Designing a Throwback Boss

[Matrota]

A while back in a 3.5e campaign I run, the party went up against some bandits who were raiding trade caravans and killed 4/5 of the bandits, as well as crippled and borderline tortured the bandit leader. The dread necromancer of the party had spared one of the bandits, sadistically calling him her "pet" and had intended to have him lead the party to the bandit hideout so they could loot it. They were taking the bandit leader back to town to turn him in for a reward. In the dead of the night, the "pet" bandit managed to escape from the rope bonds they had tied him in, stealthing away and escaping. I intend to have this character return and try to exact revenge on the party for the humiliation, having become stronger since last time they met.

This character was a 5th level rogue at the time, the party was 8th level. The party is now 11th level, and pretty well-optimized. I actually posted a thread here recently about this same group, so I apologize if it seems like I'm spamming. I want this "boss" to be challenging, which is likely going to prove difficult. The part composition is as follows:

11th level Tiefling Wizard / Initiate of the Sevenfold Veil (Some metamagic, go-to nuke is two empowered fireballs)
12th level Spellscale Sorcerer / Sandshaper (lots of metamagic, go-to nuke is empowered ocular-spell combustion. He also has an ACF that exchanged familiar for an animal companion, he chose a dire wolf after taking natural bond.)
11th level Half-Orc Barbarian / Bear Warrior (Entered Bear Warrior at 9th level iirc, so he's a bit behind)
11th level Human Alchemist (Pathfinder class)
11th level Human Monk / Kensai (Incorporates multiple ACFs and such, has some respectable damage output and a continuous blood wind magic item)
12th level Human Fighter / Elemental Warrior (Fire Element)
11th level Human Cloistered Cleric (Healing focused build, vow of nonviolence and vow of peace)

I was thinking maybe he's turned to darker powers to help him get revenge. His bandit group was loosely associated with an interplanar underground organization, and almost anything can be arranged through them-- for the right price. Was thinking a fiend of possession could possess his weapon and make it up to a +5 weapon in exchange for the souls of those he kills. Not only does he get a +5 weapon, but if/when they kill the bandit they'll loot a random +1 magic sword instead of +5, with a fiend dwelling within who's down to possess anyone at any time. Maybe I could give him fire immunity through some means as the two casters often use fire spells for damage? The sorcerer uses a lot of spells that don't allow saves. If he gets close the dire wolf will keep tripping him, so maybe something that gives freedom of movement? He won't fight them alone, but the party will likely focus fire him and I'd like him to last more than two rounds.

[Zexionthefirst]

That's some good stuff. I'm going to take a bit of a different route, and you can feel free to build upon it, or ignore it as you see fit (after all, I don't know much about your setting).

We'll call this Rouge Timothy. Timothy was scared by his time with the party. Physically, sure. But mentally as well. The things that Necromancer did to him, said to him, left him queasy and unsure of his place in the world. But what that same necromancer did to his love (another one of the bandits) left him enraged.

Spiteful, unable to think of anything but revenge, Timothy began seeking out power. After a time, some five levels filled to the brim with carnage (bonus if he sought out and killed NPC allies that the party hasn't spoken with/forgot about a while ago). And then Timothy found Granny Muriel.

Granny Muriel sensed the desperation and hatred in the young man. And she made him a deal. He would serve her and her sisters. And in exchange, they would help him get his revenge. Of course, Granny and her sisters are hags.

From a mechanic standpoint, allow the Hags to cast Grater Magic Weapon on Timothy's weapon (making is something like a +3 or +4 for the duration of the coven's spell). Perhaps the hags version of this spell offers a profaned bonus, letting it stack with the weapon's preexisting +1.

In addition, the three hags of the covey have access to scrolls of Energy Immunity which they use to make themselves and Timothy immune to fire. And, the hags have spell resistance which may or may not be noteworthy at this level. Throw in an hill giant (under the command of the hags, mainly because you have 7 players), and maybe some mook undead to serve as cannon fodder/speedbumps.

The Hags, having access to magical scrolls and items, (and maybe one of them has a class level) should be able to use prep time to insure they'll survive, and once Timothy is dead/close to death, they'll abandon him and flee.

[Crow_Nightfeath]

I can think of a couple things, namely traps and invisibility, if he can eat his hands on a scroll of greater invisibility it'll definitely get him to last at least 2 rounds, that is if the Spellcasters have see invisibility if not then they have to think of something else while he gets more or less free shots on them. And traps are a useful tool against parties that charge in/blow everything up with no regard to being cautious.