|Titles||The Headless King, Lord of Coiling Poison, the Father of Serpents, the Primal Coil, the Sundered God, the Slumbering Speaker Below, the Severed God|
|Areas of Concern|| Serpentfolk|
|Worshipers||Serpentfolk, lizardfolk, troglodytes, boggards, nagas, sorcerers with the serpentine bloodline, human snake cults, assassins|
|Domains||Chaos, Evil, Nobility, Scalykind, War|
|Subdomains||Demon, Leadership, Martyr, Saurian, Tactics|
|Symbol||Snake skull and ouroboros|
|Sacred Colors||Green, red|
|Images of Ydersius|
Source: Sanctum of the Serpent God, pg(s). 66ff
Ydersius and the serpentfolk were driven underground during the war with the Azlanti and forced to hide in the Darklands realm of Sekamina. In one of the final battles of the war, Ydersius was decapitated by the mighty Azlanti champion Savith and became known as the Headless King. Still believed to be living in two pieces, his mindless body is said to thrash in the lightless depths below, while his head has wasted away to a still-conscious skull. It is rumored that his head is buried somewhere in the Mwangi Expanse, luring the serpentfolk to that region. Whatever the exact truth may be, his clerics still receive his divine power, a sure sign that he has not followed Aroden into true death. A deity of unquenched passion and anger, the generally more cerebral serpentfolk used him as an example of all that was potent and primal in their psyche.  They believe that should his body and head be reunited, Ydersius would return to full power and lead his people on an attack against the surface world.
Savith decapitated the god and cast the head into a lake of fire and banished the body to the Darklands; to this day the god is still split in twain.
Since his fall, no gods have had relationships with Ydersius. Prior to his fall, the pantheon of Azlant stood together against the Father of Serpents, led by Abadar, Desna and Shelyn; it is extremely likely these three gods would re-engage their battle against a resurgent Ydersius.
Another enemy of Ydersius was the demon lord Abraxus whose portfolio includes snakes. This initiated a clash of power over the serpentfolk population dear to Ydersius. Even today, some serpentfolk will worship Abraxus and such heresy is not tolerated by the cult of Ydersius.
Ydersius is now split into two after the Azlanti heroine, Savith, decapitated the god. His head is hidden and his thrashing body wanders the Darklands. Ydersius is famous for his threefold poison: fang venom, toxic blood and a cloud of poison exuded from his body.
Three other servants are commonly known within the cult:
Church of Ydersius
- Older sources, such as Gods and Magic, grant Animal domain, but all modern sources are consistent in replacing it with War domain instead.
- Erik Mona et al. (2008). Campaign Setting, p. 247. Paizo Publishing, LLC. ISBN 978-1-60125-112-1
- Sean K Reynolds et al. (2014). Inner Sea Gods, p. 192. Paizo Inc. ISBN 978-1-60125-597-6
- Sean K Reynolds. (2008). Gods and Magic, p. 52-53. Paizo Publishing, LLC. ISBN 978-1-60125-139-8
- James Jacobs and Greg A. Vaughan. (2008). Into the Darklands, p. 57. Paizo Publishing, LLC. ISBN 978-1-60125-140-4
- Tim Hitchcock et al. (2010). Heart of the Jungle, p. 15. Paizo Publishing, LLC. ISBN 978-1-60125-247-0
- James Jacobs et al. (2011). The Inner Sea World Guide, p. 235. Paizo Publishing, LLC. ISBN 978-1-60125-269-2
- Tim Hitchcock et al. (2010). Heart of the Jungle, p. 41. Paizo Publishing, LLC. ISBN 978-1-60125-247-0
- Rob McCreary. (2011). Ydersius. Sanctum of the Serpent God, p. 67. Paizo Publishing, LLC. ISBN 978-1-60125-307-1
- Jesse Benner, Rob McCreary, and Neil Spicer. (2011). Bestiary. Sanctum of the Serpent God, p. 83. Paizo Publishing, LLC. ISBN 978-1-60125-307-1
- Rob McCreary. (2011). Ydersius. Sanctum of the Serpent God, p. 71. Paizo Publishing, LLC. ISBN 978-1-60125-307-1
- Jesse Benner, Rob McCreary, and Neil Spicer. (2011). Bestiary. Sanctum of the Serpent God, p. 82. Paizo Publishing, LLC. ISBN 978-1-60125-307-1
- Erik Mona et al. (2008). Campaign Setting, p. 198. Paizo Publishing, LLC. ISBN 978-1-60125-112-1
- Rob McCreary. (2011). Ydersius. Sanctum of the Serpent God, p. 70. Paizo Publishing, LLC. ISBN 978-1-60125-307-1
- F. Wesley Schneider. (2012). Artifacts & Legends, p. 48. Paizo Publishing, LLC. ISBN 978-1-60125-458-0