Aura overwhelming all; CL 30th Slot none; Weight 150 lbs.
The skull of this impossibly huge viper seems to weep darkness from its ancient fangs in an endless, pestilent breath.
More than just a powerful artifact, the Skull of Ydersius is the actual skull of a deity, the vessel through which the much-diminished god of serpents perceives and influences existence. Ydersius's clerics still receive spells from their prayers, granted by the skull regardless of its location or ownership. There is no way a mortal can block this effect. Possession of the skull grants several abilities.
Any good-aligned creature that carries the Skull of Ydersius, even via an extradimensional space such as a portable hole, gains three negative levels. These negative levels remain as long as the skull is carried and disappear when the skull is released. These negative levels never result in actual level loss, but they cannot be overcome in any way (including restoration spells) while the skull is carried. A creature that suffers from these negative levels has a 50% spell failure chance on all divine spellcasting attempts.
The only way to destroy the Skull of Ydersius is to reunite it with Ydersius's mindless, decapitated body. Doing so returns Ydersius to life, though in a weakened state. This manifestation can be combated and even slain, though it regenerates from nearly any wound short of re-decapitation. For an instant, before the whole body begins to regenerate, Ydersius's spirit is untethered from his form. If Ydersius is dragged to the Boneyard and slain in Pharasma's presence, the goddess of death can judge the serpent god, causing his body and skull to disintegrate to dust.
When the first human cultures rose upon Golarion, the empire of the serpentfolk already sprawled across the surface and Darklands. Throughout the Age of Legend, humans did battle with the serpentfolk and their dark god, Ydersius, and in the shadow of their menace forged the empire of Azlant. The wars between humans and serpentfolk escalated as the innovative humans grew mighty in the ways of magic and prophecy. Eventually, the seers of Azlant became convinced of a great doom facing their people. Blaming their reptilian enemies, they launched a crusade against the serpentfolk that led them all the way to the jungles of Garund and the subterranean city of Ilmurea. There the Azlanti hero Savith came face to face with the Ydersius himself. In the battle that ensued, Savith managed to behead Ydersius, sending his undying body thrashing into the lightless abysses of Orv, and casting his venom-spewing head into a pool of magma. Though she was mortally poisoned by the god's lethal venoms, Savith had won, and the serpentfolk empire dwindled. Though Ydersius was defeated, the serpent god is not dead, but merely locked away in his skull. There he remains, fuming endlessly as he waits to be discovered and restored to his monstrous frame and deific status.
The Skull of Ydersius is nothing less than the corpse of a deity. Those who meddle with the skull meddle in the dealings of the gods themselves.