Damn, even immortals need to watch out when fighting her, that hit die drain is nasty.D&D's Immortal Set has Demogorgon but as far as lore it mostly only lists her (she is a her in BECMI) as a high eternal of entropy and her list of epithets "Demogorgon (The Child, Bane of Souls, The Lizard King, The Dark Lady)" and goes with a slightly different look when in her true form
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(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.