From the abyss
When the Blood Storm arrived on Telara, and Akylios's spawn hauled themselves up from the seas, those mortals who were not dragged under the waves ran for their lives. Only one young scholar would not flee, for under the hideous gibbering and piteous screams she heard someone singing a song of infinite knowing.
Stealing a boat, she rowed far out to sea, where miles-long monsters churned the waters with their writhing. She tied a rock to her legs and dove overboard. The weight dragged her into the crushing deeps, closer and closer to the song. In agony she shut her eyes, and when they opened again she looked upon the singer. Akylios gave her but a portion of the dread secrets he kept, and under their weight her mind snapped, her face was worn smooth, and her name vanished from memory.
She became the first leader of the Abyssal. Like her, some inquisitive souls refuse to control their thirst for knowledge, seeking answers to questions best left unasked. Others simply seek arcane power, even if they must drag it from the darkness. Only when they face Akylios himself do they learn that such knowledge and power drove him beyond madness eons ago, and he is only too happy to lead them in the same direction.
They know your nightmares
All the Dragon Cults are horrific, but the Abyssal are the most... disturbing. Quests for gold, destruction, or even power over the dead make sense in a villainous sort of way, but the students of Akylios defy reason. They chuckle quietly outside your window at night, moving into the shadows when you come to look. They paint hideous symbols in blood upon the walls of locked rooms, or upon the faces of sleeping children.
The Abyssal are not tightly organized like the Endless Court or Golden Maw, but operate in independent cells. Individual Tidelords remain in contact, concentrating cult efforts to free Akylios and share his senseless gifts with the world. Cliques and factions rise and fall within the cult, feuding rarely. When conflict does arrive, it overturns the entire structure of the cult. Such upheavals are carefully planned by the Tidelords, for tumult pleases the lunatic dragon.
The faceless men
A secretive cabal, the Abyssal are impossible to understand. Yet every Telaran knows the Abyssal on sight by their ornate, faceless masks. A former Abyssal himself, the Faceless Man of Meridian still wears his mask, and while the Guardians decry such an abomination working with the Ascended, even the Defiant wonder what their spymaster is thinking beneath that polished silver.
The truth is, the Faceless are indeed faceless, giving up their identities in a ritual to Akylios. Each must confront the black void where the infinity of all knowledge blends into incomprehension. They cast their true names into the darkness, cut their features away, and replace their rational minds with equal parts insanity and genius. It is tempting to underestimate the Abyssal as barking loons, but remember: Akylios keeps his promise of great knowledge and clarity of thought-knowledge best forgotten, and the clarity of madness.