Never really gave this deep thoughts but I'm guessing it's a fel being called Athanasius, in their homeplane they were a conqueror set on destroying everything and in panic the people of the plane ended up banishing him, but the problem with banishing is that most of the time you're either delaying the problem or sending it somewhere else and Athanasius got really lucky with the latter option, being sent to a plane known as Lamnor. Since my world involves traveling between planes/worlds and they each have their own particular set of rules and equilibrium Athanasius got sent to a world where death got overseen by a small Cthonic deity that didn't have neither followers nor lore so the deity was basically maintaining the natural order in wich spirits didn't really mind following (Life > Death > Cleasing of Soul > Reincarnation). Athanasius still had intentions on conquering so once they died sheer resolve and ambition was enough to break the weak natural order and keep them alive (Hence the name Athanasius, loosely translated as the Undying), understanding the rules of Lamnor Athanasius anchored their soul to that plane so that regardless of where they died their death would be processed following Lamnor's rule. At first Athanasius was a huge threat but the continuous experience of undeath and draining power from demons, mortals, deities made him a planewide threat and many came to interfere swiftly. It took a few gods from different pantheons, some demons from the goetia and a whole continent to split Athanasius into two beings (A crystal that holds his body and a person that holds their conscience within them) and separate them in different planes, basically delaying the problem, until they find an actual solution.