Kostchtchie is one of the many, many alternate spellings of Koschei the Deathless, a character from Russian Mythology who ranks alongside Baba Yaga as one of their most famous and powerful characters. An epically powerful wizard who found a way to make himself immortal and invulnerable by removing his soul and hiding it inside a multilayered defense; his soul (or the moment of his death, depending on whom you ask) was hidden inside a needle, which was inside of an egg, which was in a duck, which was in a hare, which was in a chest, which was buried under an oak tree, which was on an island that vanishes with the tides. In many ways, this makes him a Lich straight out of Dungeons & Dragons, except for the fact that Koschie is also a legendarily lecherous old pervert who loves to try and get into the pants of mortal maidens.
Speaking of Dungeons & Dragons, its depiction of Kostchtchie is an altogether different beast. Demon Prince of Wrath, Ice and Frost Giants, Kostchtchie was once an incredibly ugly and bitter human man living in a stereotypical Russia-esque frozen wasteland. Pushed to the edge of his tribe, he spent many long years roaming in the wilderness, until he had a chance encounter with Baba Yaga. Once the arch-hag beat the sense into him and made him stop trying to attack her, for her own whims, she removed his soul and hid it inside of a special talisman. Made invulnerable, he returned to his tribe and conquered it, leading them into years of brutal, ever-expanding dominance even as he slowly grew to first the size of an ogre, and then to the size of a giant. Ultimately, he was slain by Gwynharwyf, a female celestial paragon, only for his soul to lodge itself into the Iron Wastes, 23rd layer of the Abyss, and slowly grow into a formidable demon prince.
Kostchtchie is mocked and jeered at even by other demons for his ugliness, stupidity and violent temperament. In return, Kostchtchie secretly seeks to obliterate all demons, as he doesn't consider himself one of them. He has a particular hatred for Graz'zt and Malcanthet. His other major goals are replacing Thrym as the patron god of Frost Giants (whom he considers himself part of) and avenging himself on Gwynharwyf. Kostchtchie's cult is legendarily misogynistic.
Because Kostchtchie is based on a Russian folklore-figure, he actually appears almost the same in Pathfinder. Major differences being that he was a powerful warlord, and a legendary misogynist, before Baba Yaga stole his soul and put it into a necklace because Kostchtchie demanded that she would turn him into an immortal and that he found his own way to the abyss without needing to be killed by a dragon and that he became a demon lord by being more proactive than the current demon lord of frost giants who spends most of his time sleeping. Also he has a burning hatred for frost giants only trumped by his hatred for Baba Yaga. It is never actually explained how he managed to get into the Abyss, so one can only assume that he raged so hard after losing his soul that he tore a hole into the fabric of the Material Plane, tripped on a branch and fell head first into Abyss.
|The Demon Princes of Dungeons & Dragons|
|Tanar'ri:||Baphomet - Demogorgon - Doresain - Fraz-Urb'luu - Graz'zt |
Juiblex - Kostchtchie - Lolth - Malcanthet - Miska the Wolf-Spider
Orcus - Sess'innek - Shaktari - Turaglas - Yeenoghu - Zuggtmoy
|Obyrith:||Dagon - Obox-ob - Pale Night - Pazuzu - The Queen of Chaos - Sertrous|
|The Demon Lords of Pathfinder|
|Demon Lords:||Abraxas - Aldinach - Andirifkhu - Angazhan - Areshkagal - Baphomet - Cyth-V'sug - Dagon |
Deskari - Flauros - Gogunta - Haagenti - Jezelda - Jubilex - Kabriri - Kostchtchie - Lamashtu
Mazmezz - Mestama - Nocticula - Nurgal - Orcus - Pazuzu - Shax - Shivaska - Sifkesh
Socothbenoth - Urxehl - Xoveron - Yhidothrus - Zevgavizeb - Zura
|Nascent Demon Lords:||Daclau-Sar - Izyagna - Kro'akoth - Menxyr - Murnath - Nightripper - Ovonovo - Shamira |
Sithhud - Treerazer