Spoiler: Description

The booming voice explodes over the mountaintop and echoes endlessly.

"And we now welcome our twelfth candidate...No less than...
The Glorious NERO, Fireshaper, Lord of the Pain Harmonics, Prince of Creative Destruction, King of Fiery Passion and Emperor of the Artistic Arson! And his Ingenuous Assistant, the raven BRITANNICUS!"

The public nods approvingly at the description. The members of the jury keep their faces stern, but several of the telepathically bonded kobolds start to write down notes frenetically.

The Amplified voice of the anchor resumes on his rant.
"Here is the name of Nero's presentation:
A synesthetic variation over the third symphony of Franjof Hidn, "The five tongues of Tiamat" (n3 op.42). It seems our bold new candidate does not shy away from tackling one of the most difficult and renown masterpieces of the late Red Dragon composer! Let us see if he manages to win over the jury with such a daring execution. Image, please."

At this command, technical staff buzz about behind the scene. The relevant scroll of Scry Location is produced, while at the same time the small silver mirror is Levitated into place for all the mountaintop amphitheater to see. As the Shrink Item is dispelled, the slightly concave screen regains its 16x9 feet dimension, while the scroll is triggered. To finish off, an Amplify is cast on the display. The public mumbles with satisfaction as the connexion is established. A topview of a small town appears; Halflings can be seen hustling and bustling about, oblivious to the magical eye hanging midair above them.

"Ah, here we are. Nero, are you receiving us?"
"...Perfectly, Mr Anchor, I have just finished installing my own communication set up. I am currently flying over the sensor, hidden for the moment within a bank of clouds."
"Dear members of the jury...Mr Anchor...dear public gathered today...before I begin, let me introduce you the Halfling city of Kirinth."

Impatients growls can be heard among the younger spectators.
"I know you all long for admiring my extraordinary performance. I simply wanted to point out the particular regime of these Halflings, who have been eating a very spicy food for generations. This has changed the tone of their voice in a way I had been looking for for years. Now, I will let my faithful Britannicus relay me at the sensor."
The mandatory round of raucous howling is produced by the audience. A few Wyrmlings shift in place with excitation.

A Dragon-shaped shadow suddenly flashes below the sensor; a bell immediately tolls in answer. Halflings are seen hurrying around, panicked. A raven circles just below the magical eye, looks towards it, and proudly squeeks:
"The first movement of the symphony will begin by the classical auditory performance. Please rotate the sensor to the east".

Nero is seen landing on a forge, destroying it by his weight. He seizes a handful of white-hot anvils, oblivious to the heat, and throws them in a well. At the same time, he starts to sing; the hissing of the water perfectly highlights the deep, rich tones of his voice. He his a marvel to hear. A judge makes a knowing nod.
A volley of arrows rush towards him, and Nero lets them land on his wings. Though they slip harmlessly against his scales, the drumming sound they produce is taken advantage of on the spot to accompany the high-pitch aria that constitutes the leitmotiv of the first movement.

The symphony goes on. At the crescendo that precedes the final chord, Nero starts to hum in rythm, without stopping to sing. A squad of guards hesitates at this music, and slows its charge for a second; then the obvious sight of Halfling corpses everywhere seems to harden its will again. The black shape of Britannicus suddenly adds:
"Nero has chosen to reinterpret the final crescendo as a more complex fugue."
Several Old Dragons frown at this bold personalisation of the masterpiece.Their disapprovement disappears when the four guards suddenly impale each other; the noise of their bodies falling down joins with the voice of Nero to triumphally produce the final chord of the melody.

The black raven goes on commenting in creaking Draconic.
"For the second movement, prepare for visual interpretation...turn the sensor to the southwest, please."
Nero falls onto a sturdy cottage, as yet unharmed. Ripping apart the roof, he reveals several screaming families. Unpleased at this sound, he roars at them, and the Halflings suddenly all cower into more discrete sobs. The fire in the hearth grows to a huge blaze; the poor creatures accordingly flee away from the flame into the street.
"This motion evokes the flame of inspiration that ignites Franjof Hidn's mind, and the numerous compositions he produces spread out in all the draconic world.", says Britannicus. Nero lands before the ruined house and, just before a ballista bolt strikes at his head, a pair of shimmering white wings appears and wrap around the Dragon. The constrast with the flaming house behind him is striking.
"Of course, many look down on the master's creation, but their scorn does not attain him."

The second movement comes to and end, and the third movement begins. Some are puzzled among the viewers: so far, Nero has not unleashed his breath once, even though he deems himself the Emperor of the Artistic Arson. Moreover, the Halfling being butchered don't display any particular spice-induced feature. This is, of course, at that moment that Britannicus adds in one final comment.
"The final
alla breve comes near. Please pay a particular attention to this part. Rotate the sensor to the north."
The last remaining Halflings guards have gathered behind a small stone parapet, readying a volley of arrows. An older guard, apparently a mage, casts a spell that every Red Dragon in the amphitheater recognises at once for having seen it many times: Energy Resistance.
Suddenly, Britannicus bolts towards the soldiers, and, to the surprise of the jury, unleashes a torrent of fire upon them. At once, they turn towards him and unleash their volley. Seizing the opportunity, and singing the accelerated last measures, Nero faces them. He inspires deeply and exhales not fire, but a transparent fluid that envelops the fighters. The mage is seen cursing and blasting. At the same time, the Red Dragon lets loose an all-powerful yet melodic roar of rage, scoring the penultimate note of the symphony; and spews out another breath less than one second after the first. There again, eyebrows (or what passes for them in the case of a Dragon) raise. The cone of fire that comes forth entirely fills the vision provided by the magical screen.
And the eerie miracle appears: The screams of agony of the cooked Halflings don't ring a La, as every Red Dragon has heard more than once, but a La sharp! The symphony is complete! The amphitheater is immediately filled with enthusiastic roars. The Wyrmlings have not seized the nuance but appreciate a good massacre when they see one; the Old ones and the jury are wholeheartedly amazed.

A minute later, Nero appears, beaming, facing the screen (Britannicus having been unfortunately reduced to ashes by the previous fire blast).
"Dear members of the jury, Mr Anchor, dear public...this was my participation to the Red Dragon of the Decade Championship, in the Juvenile Category."

Spoiler: Build & Overview

Nero is a Juvenile Red Dragon with the Passion's Flame sovereign archetype (Dragons of Eberron p.30). He has 1 level of Sorcerer, 3 levels of Virtuoso (Complete Adventurer), and 6 levels of Sublime Chord (Complete Arcane).
He sports a Familiar, the raven Britannicus. Nero and Britannicus speak Draconic; Nero also speaks Ignan and Common.

The sovereign archetypes exchange a dragon's ability to cast spells from the Cleric spell list and from certain Cleric domains (Chaos, Evil and Fire for Red Dragons) in exchange for other benefits. The Passion's Flame sovereign archetype exchanges these domains against the Passion and Madness domains, regains the Cleric spell list, gains Perform as a class skill of Dragon HDs, and the ability to Rage once per day per 4 Dragon HDs.

CR Str Dex Con Int Wis Cha Notes
10 29 10 21:19+2 14 15 16:14+2 +2 Con item, +2 Cha item
12 29 10 23:21+2 14 15 16 +4 Con item, +2 Cha item
14 29 10 23 14 15 19:16+3 20 HD +1 Cha increase, +4 Cha item
16 29 10 25:23+2 14 15 21:19+2 Con +6 item, Cha +2 tome
18 29 10 26:25+1 14 15 24:21+3 24 HD Cha increase, Cha +6 item, Con +1 tome
20 29 10 26 14 15 26:24+2 Cha +4 tome

CR Class Base Attack Bonus Caster Level Fort Save Ref Save Will Save Skills Feats Class Features
10 Passion's Flame Sovereign Archetype Juvenile Red Dragon (16 HD) +16 3 +15 +10 +12 Perform (Oratory) 19, Listen 19, Spot 19, Intimidate 19, Diplomacy 19, Bluff 5, Knowledge (Arcana) 10, Knowledge (History, the Planes, Local, Nature, Religion) 5, Appraise 5, Spellcraft 3 (cross-class) Flyby Attack (MM), Power Attack, Imperious Command (DoTU), Heighten Breath (Draco), Quicken Breath (Draco), Enlarge Breath (Draco), .

Skill Tricks (CSco): Never Outnumbered, Clarity of Vision, Collector of Stories.
Breath Weapon (Fire damage 8d10, 40ft cone, save DC 22), Immunity to fire, Vulnerability to cold, Locate Object (4/day), Rage (4/day)
11th Sorcerer 1 +16 4 +15 +10 +14 Perform (Oratory) 19, Listen 19, Spot 19, Intimidate 19, Diplomacy 19, Bluff 5, Knowledge (Arcana) 10, Knowledge (History, the Planes, Local, Nature, Religion) 5, Appraise 5, Spellcraft 3 (cross-class), Profession (Astrologer) 4
12th Virtuoso 1 +16 4 +16 +10 +16 Perform (Oratory) 21, Listen 19, Spot 19, Intimidate 21, Diplomacy 19, Bluff 5, Knowledge (Arcana) 10, Knowledge (History, the Planes, Local, Nature, Religion) 5, Appraise 5, Spellcraft 3,Profession (Astrologer) 6 (cross-class) Melodic Casting (CM) Bardic Music (Fascinate), Virtuoso Performance (Persuade)
13th Virtuoso 2 +17 5 +16 +10 +17 Perform (Oratory) 22, Listen 19, Spot 19, Intimidate 22, Diplomacy 19, Bluff 5, Knowledge (Arcana) 10, Knowledge (History, the Planes, Local, Nature, Religion) 5, Appraise 5, Spellcraft 9, Profession (Astrologer) 6
14th Virtuoso 3 +18 6 +17 +11 +17 Perform (Oratory) 23, Listen 19, Spot 19, Intimidate 23, Diplomacy 19, Bluff 5, Knowledge (Arcana) 10, Knowledge (History, the Planes, Local, Nature, Religion) 5, Appraise 5, Spellcraft 15, Profession (Astrologer) 6 Virtuoso Performance (Sustaining Song)
15th Sublime Chord 1 +18 11 +17 +11 +19 Perform (Oratory) 24, Listen 22, Spot 21, Intimidate 23, Diplomacy 19, Bluff 5, Knowledge (Arcana) 10, Knowledge (History, the Planes, Local, Nature, Religion) 5, Appraise 5, Spellcraft 15, Profession (Astrologer) 6 Practised Spellcaster (Sublime Chord) (CArc) Bardic Music, Bardic Lock
16th Sublime Chord 2 +19 12 +18 +11 +20 Perform (Oratory) 25, Listen 24, Spot 24, Intimidate 23, Diplomacy 19, Bluff 5, Knowledge (Arcana) 10, Knowledge (History, the Planes, Local, Nature, Religion) 5, Appraise 5, Spellcraft 15, Profession (Astrologer) 6 Song of Arcane Power
17th Sublime Chord 3 +19 13 +19 +12 +20 Perform (Oratory)26, Listen 26, Spot 25, Intimidate 24 (cross-class), Diplomacy 19, Bluff 5, Knowledge (Arcana) 10, Knowledge (History, the Planes, Local, Nature, Religion) 5, Appraise 5, Spellcraft 15, Profession (Astrologer) 6
18th Sublime Chord 4 +20 14 +20 +12 +21 Perform (Oratory) 27,Listen 27, Spot 27, Intimidate 25 (cross-class), Diplomacy 19, Bluff 5, Knowledge (Arcana) 10, Knowledge (History, the Planes, Local, Nature, Religion) 5, Appraise 5, Spellcraft 15, Profession (Astrologer) 6 Invisible Spell (CS)
19th Sublime Chord 5 +20 15 +20 +12 +21 Perform (Oratory) 28,Listen 28, Spot 28, Intimidate 26.5 (cross-class), Diplomacy 19, Bluff 5, Knowledge (Arcana) 10, Knowledge (History, the Planes, Local, Nature, Religion) 5, Appraise 5, Spellcraft 15, Profession (Astrologer) 6
20th Sublime Chord 6 +21 16 +21 +13 +22 Perform (Oratory) 29,Listen 29, Spot 29, Intimidate 28 (cross-class), Diplomacy 19, Bluff 5, Knowledge (Arcana) 10, Knowledge (History, the Planes, Local, Nature, Religion) 5, Appraise 5, Spellcraft 15, Profession (Astrologer) 6 Song of Timelessness

Spoiler: Spellcasting abilities

The following table include the bonus spells per day granted by a high Charisma score.

Caster Level/Spells per Day/Spells Known
CR CL 0lvl 1st 2nd 3rd 4th 5th 6th 7th 8th 9th
10 3 6/5 6/3 - - - - - - - -
11 4 6/6 7/3 4/1 - - - - - - -
12 4 6/6 7/3 4/1 - - - - - - -
13 5 6/7 7/4 5/2 - - - - - - -
14 6 6/7 7/4 6/2 4/1 - - - -
15 11 6/7 7/4 6/2 4/1 3/3 1/1 - - - -
16 12 6/7 8/4 6/2 4/1 3/4 3/2 - - - -
17 13 6/7 8/4 6/2 4/1 4/4 3/2 1/1 - - -
18 14 6/7 8/4 7/2 5/1 4/4 4/3 2/2 - -
19 15 6/7 8/4 7/2 5/1 5/4 4/3 3/2 2/1 - -
20 16 6/7 8/4 7/2 5/1 6/4 4/4 4/3 3/2 -

Spells Known:

Cantrips: Dancing Lights, No Light (BoVD), Flare, Prestidigitation, Detect Magic, Read Magic
1st level: Mage Armor, Shield, Unseen Servant, Blood Wind (SC)
2nd level: Scintillating Scales (SC), Wings of Cover (RotD)
3rd level: Sonorous Hum (SC)
4th level: Freedom of Movement, Fire Shield, Fugue (SC), Channeled Pyroburst (PHBII)
5th level: Dragon Breath (SC), Dispelling Breath (SC), Stunning Breath (SC), Arcane Fusion (CM)
6th level: Otto's Irresistible Dance, Imbue Familiar with Spell Ability (SC), Antimagic Field, True Seeing (in the remaining 7th level spell slot)
7th level: Arcane Spellsurge (DM)

Spoiler: Sources

Book of Vile Darkness (BoVD)
Cityscape (CS)
Complete Adventurer (CAdv)
Complete Arcane (CArc)
Complete Mage (CM)
Complete Scoundrel (CSco)
Draconomicon (Draco)
Dragon Magic (DM)
Dragons of Eberron (DoE)
Drows of the Underdark (DotU)
Player's Handbook II (PHBII)
Races of the Dragon (RotD)
Spell Compendium (SC)

Spoiler: Build explication

The idea behind Nero is to combine the metabreath feats and the metabreath spells to give a Red Dragon a very powerful and versatile breath weapon. The Red Dragon provides the fire immunity, his breath weapon the at-will fire damage. For this, high-level arcane spells were needed. Since the villain must be usable throughout the campaign at different CRs, we can't simply pick the same Red Dragon at Juvenile, Young Adult, Adult and Mature Adult stages, since that would mean the Dragon has aged several decades between two fights with the party. Thus, we need class levels to obtain the needed spells. Capitalising on the 3 levels of innate Sorcerer casting of a Juvenile Red Dragon seems obvious, but to get to 5th level spells (Dragon Breath, Dispelling Breath) before CR 17, which would be a bit late, we can't use Sorcerer 10. A fast-casting prestige class then? Enter Sublime Chord. Moreover, the added versatilty of the remaining high-level arcane spells gives Nero tools to deal with fire immunities and alleviate the common weaknesses of Dragons. The Sublime Chord class then naturally shapes the rest of the build by its prerequisites, and dictates the flavour of the villain: a megalomaniac and destructive performer who sees art in the devastation he sows.
As an added perk, by Sublime Chord 10 Nero will gain the Song of the Arcane Fire, which produces fiery destruction proportional to the quality of his chant...what could better fit the character?

Spoiler: How to play Nero

As always with Dragons, Nero is a villain both very powerful for his CR and very flexible. Depending of the level of optimisation of the party, Nero can be played as:

-A (flying)melee beatstick;
-A buffed up melee monster;
-A debuffing, buffed up, action-economy-exploiting, familiar-for-extra-actions-using, formidable opponent!

In RP, to justify for the level of power chosen, the old classic is "Ha, I don't even have to use all of my might against these worms!". As Nero is an artist, this can be spun into a "These weaklings don't deserve to see my chef-d'oeuvre!" Now let us see how those roles are fulfilled.

-The (flying) melee beatstick: Nero has, as soon as CR 10, access to Power Attack and 6 natural attacks, a bite, two claws, two wing buffets and a tail slap. With his Str 29 and his high BAB, he can deal tremendous physical damage. The addition of Flyby Attack makes him much less susceptible to counterattacks and allows him to dive for double damage. His HPs are high and his saves intimidating. A party that merely relies on physical damage willl have a hard time beating him. Once or twice during the battle, a regular breath attack will deal some damage ranging from significant to barely noticable, depending of the level. Moreover, if appropriate (ie, a beautiful violin in Nero's hoard is destroyed during the fight), Nero can Rage.

-The buffed-up monster: Nero has access to a good amount of buff spells. With his excellent senses: Listen/Spot=CR +13 (9 ranks, Alertness, Aid Another) most of the time, Darkvision 120ft, quadruple low-light vision, Blindsense 60ft, Nero has high chances to roll for initiative while the PCs are still far away, in case of a moderately optimised party (i.e., no Love's Pain or Scry'n Shivering Touch). This means he can set up a consequent amount of defensive spells: Freedom of Movement, Fire Shield against cold damage, Mage Armor and Shield, True Seeing. Scintillating Scales pump the touch AC to 24, which will make casters struggle before they use Quickened True Strikes.
A Blood Wind can allow for a full attack at 20ft, and since Nero occupies already 10 square feet, it makes for a good area attack. A Quickened, Enlarged, Heightened to the maximum breath attack will be usable only once per battle, but will cover the whole map for an impossibly high save DC, as a free action (tip: use it during a Flyby Attack). The damage is somewhat low (8d10 from CR 10 to 20) and is fire-typed, but it's a free action!
The ace in the hole is, of course, Antimagic field. As shown by Vaarsuvius, a raging Juvenile Red Dragon protected by an Antimagic Field can Power Attack at maximum strength and use a Dive attack on the poor caster, dealing tremendous damage while avoiding to be locked in melee.

-The "all of the above" Dragon: Nero played to the maximum of his abilities. He uses Imbue Familiar with Spell Ability to give spells to cast to Britannicus, thus gaining extra actions: an Arcane Fusion containing Freedom of Movement and Mage Armor, two spells that Britannicus can cast on Nero for one standard action. Blood Wind. Britannicus always remains on Nero's shoulder, thus gaining, with the Share Spell ability, the use of Fire Shield, Shield, but also Dragon Breath, Antimagic Field and Arcane Spellsurge.
As to what Nero does: he buffs up with Arcane Fusion (Fire Shield + Shield), dishes out Otto's Irresistible Dance during Flyby Attacks, casts Sonorous Hum + Fugue for an area debuff (even on a successful save), and does this twice as fast with Arcane Spellsurge combined with Invisible Spell!
An Intimidate action combined with his humongous modifier, the Imperious Command feat and the Never Outnumbered skill trick allows for a powerful one-shot area debuff, and nonmagical at that.
But the Red Dragon is the Emperor of the Artistic Arson, and a fire-type monster before all. How does he fit the theme already? By his breath weapon tactics. He can cast Dispelling Breath (to get rid of many pesky things, including fire immunities), Stunning Breath, Blinding Breath, use the Heighten Breath, Quicken Breath and Enlarge Breath (which stacks as shown p.66 of the Draconomicon), all of these on both his native breath weapon and the one given by the Dragon breath spell. He stacks up metabreath feats to the maximum, so the reloading turn of his breath weapons far outruns the duration of the battle. But who cares? He can, and will, cast several Dragon Breath spells per battle!

Spoiler: CR 10 overview

A Red Dragon is terrorising the region...what more to say? At this moment, Nero is almost a plain vanilla Red Dragon. He has no class levels yet, but is already a worthy opponent for a level 10 party. The party will come after him in his lair, and as the PCs gain intelligence about him, they better understand his artistic perception of himself. He only steals paintings, music instruments and other chef-d'oeuvres. He loves singing as he lays fiery devastation everywhere, and he sees the destruction by fire as a form of creation.
As a raging beatstick, we have 200 HP, a Dive Power Attack bite attack at 72 damage (a Bard with Con 16 has 68 HPs), the Bloodwind spell to get a full-round action at 20ft a debuffer, an Intimidate modifier of +28 who will send everyone within 10ft cowering. The breath attack dishes out (as a free action!) 44 fire damage, in an infinitely long cone (by stacking Enlarge Breath) and a Ref save of 28, or 32 in a rage! Who can roll that at level 10? A 10th level Rogue nabs Ref +15 if he's lucky....but Cowering ennemies don't get Dex to Ref saves. As a buffer, Mage Armor and Shield give AC 32, preventing Power Attackers from being too effective.
Skill points are spend in an equally useful (Perform, Listen, Spot, Intimidate, Diplomacy, Bluff, Knowledge: History, Spellcraft, Skill Tricks: Imperious Command and Clarity of vision) and fluffy (Appraise, all the other Knowledges, Skill trick: collector of stories) way.
Prestidigitation keeps Nero's hoard clean, while the various light and fire-related cantrips can accentuate the dramatic effect of a performance at a key moment.
The saves are very high, the mobility is good...what to fear? Since Nero does not have any SR, no save stuff like Ray of Exhaustion can bring him down very quickly. The casters must be his first target.

Spoiler: CR 15 overview

Nero has been beaten last time and he does not like that. Especially if the party was containing a Bard, or another class with ranks in Perform: Nero can make them his archnemesis. He will pursue the party, never attacking directly. Instead, he will sneak into vilages at night, use his Fascinate and Persuasion bardic musics to lure people out of their houses on the outside, remaining out of view while he enjoys his Darkvision. When enough people are gathered, a Quickened Extended breath toasts them all at once, while minimising the damage to the buildings. Nero can then plunder for artistic items, jewelries, fine sculptures, etc. His Locate Object spell-like ability is very useful for this. He will, however, always let behind a single flaming candle to sign off his misdeed, in order to enrage his rival.
When the battle occurs again, Nero is Sorcerer 1/Virtuoso 3/Sublime Chord 1.
He has gained in HP (244) and Fort and Will saves (+17/+11/+19). His melee prowesses are essentially the same. He can still put everyone to Cower, since his Intimidate skill is still maxed.
What has changed? As a buffer, he now boasts Scintillating Scales for a +15 in Touch AC (touch AC 24, a ranged touch attack from a 15th level Wizard with Dex 16 has a +10 modifier), Freedom of Movement, and Fire Shield to counter his Cold Weakness. Wings of Cover negates the Shivering Touch. As a debuffer, Sonorous Hum + Fugue is a serious effect, imposing -2 on attack rolls on a successful save, and with the Aid another from Britannicus, a Perform DC of 30 to make everyone nauseated is almost assured with a +24 modifier. Fascinate and Persuasive Song have mainly out-of-battle uses, as shown above. Remember how Smaug slowly charms Bilbo over with his voice?
As a breather, Dragon Breath spell now appears! It only deals 5d8 damage, Ref half, but with Heighten Breath, Quicken Breath and Enlarge Breath Nero can use the breath on the turn he casts the spell for Ref save DC 25, and cover all the battlefield...and with Share Spell, Britannicus immediately can do the same! Twice as much payback (with a lower save and range, but still)! Flyby attack, cast Dragon Breath in the middle of the movement, use the breath weapon immediately; Britannicus has readied an action to unleash his newly acquired breath weapon at that moment get away from danger in the same turn.
Unseen Servant spell is used for cleaning Nero's hoard effortlessly and faster than Prestidigitation. Melodic Casting allows to substitute Concentration for Perform for spell-related checks, freeing skill points for more interesting areas (such as Profession: Astrologer).
The AC has become low now, however, and the Ref save too; but Nero can fight at a distance better than before, which helps put him away from danger.

Spoiler: Cr 19 overview; sweet spot

Nero is now a worldwide famous Dragon performer (or infamous, depending on whom you ask). He breathes fire longer and farther than anyone else, he can scorch cities within a few seconds. His musical prowess synergises with his arcane powers. No one can rival him...but the Bard PC, of course! An intricate trap will lead the party into challenge where Nero can defy him one on one on a "Perform" duel. The contestants are free to do whatever they want with a country each to produce the best art form. An assembly of Passion's Flame Red Dragons, will be the judge, of course. The PCs then embark on raising an army out of their assigned country to defeat the evil conclave, while Nero carefully rampages through his.
The Red Dragon has improved in all areas, and no small thanks go to Arcane Spellsurge. All spells are quickened for free. This is shared with the raven. The Invisible spell metamagic feat does not change the spell's level but turns its casting into a full-round action, and then into a standard action again with Arcane Spellsurge. The addition of Antimagic Field makes him a potent melee fighter once again. Since the spell is Shared with Britannicus, a Disjunction would need to bypass the two of them. If facing a squishy Wizard with a bazillion buffs that make him untouchable/unkillable/Shapechanged etc, Nero casts Antimagic Field as a swift action, makes the usual Flyby Attack dive Power attack, and laughs.
For debuffs, Intimidate most likely won't work anymore, due to everyone being magically immune to fear...except within the range of an Antimagic Field! Otto's Irresistible Dance as a swift action just before a quickened breath will give a -10 to Ref saves to that Rogue, and generally make him harmless for the whole battle. Yes, it is Mind-affecting, but what about a swift action Dispelling Breath on the previous round? A targeted dispel check on everyone that fails the Ref DC 38 save, to eliminate this as well as any Energy Immunity/Fear immunity/Flight/Magic need to identify who is under the effect of which spell, just dispel everything. Though to be sure, Spellcraft 15+Aid another+Int modifer allows to recognise a 9th level spell by a roll of 8 or less, if the PCs are far away Nero can take 10. And Nero can cast Sonorous Hum as a swift action, and then Invisible Fugue as a standard action, and then forget about it. Hitting Perform check 40 to make the PCs attack one another is easy with a +37 modifier. The Song of Timelessness, as the other Bardic Music effects, is mainly used out of battle for Nero to freeze from afar a performing dancer, so that he can study his technique very carefully by a sort of slow-motion mode.
For buffing, the trick mentioned earlier about Imbue Familiar with Spell Abilty and Arcane Fusion allows Britannicus to buff Nero by two spells at once, for a swift action! Rage keeps working in an Antimagic Field. Britannicus casting Bloodwind, in addition to a swift action casting and a standard action casting from Nero, allows for three 20ft reach full attacks in a round!
For blasting, Nero can cast Dragon Breath and Invisible Dispelling Breath in the same turn, and finally blast in for a free action immediately (7d8 damage)- and if he has not used it yet, he can fire his regular breath weapon in the same turn (+8d10 damage), also as a free action! Britannicus still gets the Dragon Breath as well. Against an army of mooks, Nero can, by sufficiently stacking Enlarge Breath on a Dragon Breath spell, deal 7d8 fire damage with a DC 38 Ref save on an area as big as a continent, wiping out every non fire-immune creature of 4HD or less or so. He then dismisses the spell, and still has his regular breath attack ready.
What to do with move actions? Either keep strafing around with Flyby attacks, so that Nero can't be cornered by the PCs; or use Song of Arcane Power for a +4 to CL when the need arises, like pumping the duration of, and the difficulty to dispel, both the Fire Shield and Shield spell contained into a single Arcane Fusion; or cast Channelled Pyroburst with the 2-rounds casting time (decreased to 1 round with Arcane Spellsurge) for when there is a spare standard action as well: 10d10 fire damage in a 20ft burst. It slowly adds up.
Nero needs the Dispelling Breath and the added action economy to break through, however. He is vulnerable to dispelling himself, though less than other dragons since he can at this point simply cast an Antimagic Field, Rage, and still be quite threatening. But he needs more than one spell per round to keep up with the PCs. At these levels, his AC, saves and lack of SR won't compensate his high (336) HPs if he can't dictate the momentum of the battle.

Spoiler: CR 20 overview

Nero simply adds Stunning Breath and True Seeing to his list of spells known, ensuring illusions won't fool him anymore, and Stunning Breath is one more area impossibly-high save or suck effect.