Y'ulea is a three-souled serpent god with a body made of light and fire of which no blade or hex could touch and magic had barely any effect on it. He constructed the celestial artefact known as the Star Crucible. Archaon encountered and battled this entity long before coming to the Mortal Realms.
Archaon forged an alliance with the Tzeentchian Sorcerers of Zyr and a cabal of Slaaneshi Spell-sirens in order to steal the artefact. He ordered his allies into the battle with the god and when they perished, the Everchosen used their souls to weave a sorcerous cage of prismatic mirrors around the beast. Y'ulea beast's body exploded in a thousand brilliant hues as the prisms channelled and dissipated its incorporeal form, allowing Archaon to claim the artefact.