|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|
|Home||Darklands; Sydrixus, Maelstrom|
|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.
|Avatar of Ydersius|
(chaotic, evil, extraplanar)
Source: Sanctum of the Serpent God, pg(s). 54
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, the mighty Azlanti champion Savith decapitated Ydersius, who then became known as the Headless King. Still 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 serpentfolk to that region. Whatever the 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.
Since his fall, no gods have had relationships with Ydersius. Prior to his fall, the pantheon of Azlant led by Abadar, Desna, and Shelyn stood together against the Father of Serpents, and it is extremely likely that these three gods would re-engage their battle against a resurgent Ydersius. He has also long been opposed to Yig, who has turned his attentions elsewhere after Savith defeated Ydersius.
Another enemy of Ydersius was the demon lord Abraxas whose portfolio includes snakes. This initiated a clash of power over the serpentfolk population dear to Ydersius. Some serpentfolk still worship Abraxas, and the cult of Ydersius does not tolerate such heresy.
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
The Skull of Ydersius, a powerful artifact, can be attached to his headless body to allow him to return to full power. Destroying it would permanently kill Ydersius, but this is an almost impossible task. For instance, one method requires Ydersius' manifestation to be dragged to the Boneyard and slain in the presence of Pharasma for her immediate judgment, which would cause him to disintegrate.
- 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
- 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
- James Jacobs. (2016). The Elder Mythos. In Search of Sanity, p. 73. Paizo Inc. ISBN 978-1-60125-882-3
- 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. 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
- James Jacobs. (2011). Beyond Serpent's Skull. Sanctum of the Serpent God, p. 60-61. Paizo Publishing, LLC. ISBN 978-1-60125-307-1