Well, here's my go at it:
I am totally stealing Red Fel's format for this.
Illius, the Broken Chord
There are those in this world who search for perfection - for the sublime beauty upon which nothing can improved. It is a noble cause, and consumes lifetimes of study and mastery. Eventually, it may be found, and is treated as a paragon of all that is good and right in its discipline. It shouldn't be so surprising that there exists a Paragon of Flaws, then. But somehow, it always is.
It began, of course, with a bard.
Milenia ago, when the first powers of magic were being discovered, a wandering minstrel by the name of Illyahr began to notice that his music was having an effect on those who paused to listen. He realized that this wasn't some mere fascination, or adoration, but rather than his songs plucked the heartstrings of his crowds and could magically alter their perceptions and beliefs. Pursuing his study further, he discovered the means to whisper sweet suggestions in the ears of the masses, and through this method conscripted populations to find their most talented performers and send them to him for study. Illyahr imparted his initial teachings unto them, and so founded the bardic tradition.
Throughout his lifetime, he saw this new power spread far and wide across the continents, and was content with the growth of his discovery. He founded a great school where those of the highest caliber - like himself - could tease out new secrets from the craft. Discovering greater and greater performances, Illyahr's ambition began to reach higher and further than those of his colleagues. He quickly outstripped their skill with discovery, making dozens of refinements to the craft in the same time that they would produce one. And eventually he asked, "What is the perfect chord?"
That day marked a dramatic change in the master bard. He closed himself off from the outside world, secluding himself with his instruments and notes, and sought to find the resonant frequency of the universe, from which all other sound is born. His friends would stop and listen by his tower, as soul-wrenchingly beautiful notes poured out of his window, only to be paused halfway through with cursing and cries of, "No, no, that's the wrong sound entirely!" Years passed, and eventually the school fell into disrepair and the others left to found their own places of learning or accept great positions of acclaim throughout the lands.
Illyahr began to be known as a crazy hermit rather than a brilliant teacher, and in a fashion similar to his reputation his mind began to degrade as well. His passion turned to obsession, and he would forgo food or water for days when tuning a new chord. Nearing the end of his life, his body emaciated and weak, he had a sudden flash of inspiration. He returned his fingers to the strings one final time, readied his hands, and struck his last chord. And everything trembled at its majesty and perfection. The chord rolled over the land, and all other noise ceased and every person turned to face the direction of its origin, knowing that Illyahr had succeeded. The beauty and relief flooding the aged bard was too much for his old heart to handle, and he died happy on the spot. It was a shame, really, then that in his death one finger twitched and an imperfection was added to the sublime chord. Ah, but mortals can only withstand so much...
That one flaw corrupted the Perfect Chord, and in its corruption it called upon the energies of the Far Realms. The broken note took the form of a massive incorporeal swarm, repeating its flawed notes over and over in attempt to return to perfection, instead falling further and further into dissonance and degradation. Named for its creator, Illius now seeks to have its discordant chords played and voiced throughout the multiverse, to corrupt it and become its new resonant frequency.
Illius seeks to cripple and bend the universe to match its own twisted self.
Echoes of Corruption
Living beings begin to voice the Broken Chord, becoming twisted and broken.
Faint:A quiet echo at the base of the skull. Living creatures must make a Will Save (DC 5) every hour or start humming the tune. This does not impede talking or spellcasting, and lasts until the save is made. Living creatures must make an additional Will Save (DC 2) each day or suffer a -1 morale penalty on all rolls due to the distracting background noise.
Moderate: The echo crescendos. The Save DC to avoid humming increases to 10, and while humming spellcasters must make a DC 15 Concentration check to cast a spell with a Somatic component. The Save DC against the morale penalty increases to 5 and the penalty increased to -2.
Strong: The chord reverberates. The humming changes to singing, and the Save DC to avoid it increases to 15. While singing the Broken Chord, attempting to talk requires a DC 15 Concentration check or everything comes out as the Chord. While singing, the Concentration check DC to cast spells with somatic components increases to 25. If a living creature fails every Will Save against singing in a single day, it gains the Aberration Subtype until it makes every Will Save against singing in a single day. They are still treated as though they had their originla type and subtype. Those with the Aberration subtype automatically fail the saves against the morale penalties. The Save DC against the morale penalty increases to 12 and the penalty increases to -3.
Overwhelming: The chord resounds. The singing changes to shouting, and the Save DC to avoid it increases .to 20. While shouting the Broken Chord, attempting to talk requires a DC 20 Concentration check or everything comes out as the Chord. While shouting, the Concentration check DC to cast spells with somatic components increases to 35. Aberrations who fail their every save against shouting for an entire day go Insane. The Save DC against the moral penalty increases to 15 and the morale penalty increases to -4.
In addition to the normal malefic property common to all Elder Evils, Illius has Cosmic Dissonance.
Cosmic Dissonance: Any living creature with the within the range of this Malefic Property is ripped apart by the discordant noise. Such a creature must make a Will save (DC 10 + 1/2 Illius' HD + Illius' Charisma modifier) or become Confused by the maddening noise. A creature that saves against this effect is immune to the dissonance for the next hour. A Confused creature instead becomes Insane. An Insane creature dies.
Any creature within range of this Malefic Property must save against this ability every 10 rounds. If they successfully save against it, you are immune to the effect for the next 10 minutes but do not remove any lesser effects inflicted by this effect.
Illius still believes itself to be the Perfect Chord, and will attempt to reach out first to the minds of the greatest Bards to share itself with them when its Faint sign manifests. Illius will manifest itself on the Ethereal Plane when its Strong Sign appears, and it will generally move towards the largest population centers of a region. It does not concern itself with strategy, and is largely unthinking in its pursuit of its goals.