He served Prometheus well in his life so he is rewarded in death. By a macabre way of immortalilty.
"You have done well to serve me during your mortal life so I reward you by letting you serve in your now immortal life as well." He spoke to the snow leopard when he freshly arrived in the afterlife.
"Your new occupation is quite similar to your old one: you will escort the living over to death. Just a bit more literally from now on." Laughter falls over the Styx which he now has to ferry over for eternety.... at least for now. Prometheus might be a godly entity but he isn't prone from making mistakes which he did by lifting only the constraint of mortality without attaching him to his own controlling.

Until Kjartan finds that out he will do his duty.... and escort poor souls over to the void. Some of them familiar, so familiar that he remembers how he sent some of these souls onto their voyage to the void, now passing over the Styx.
A wicked and straining but entertaining program for the ascended entity.

I got the idea for this while playing the game Spiritfarer. Can only recommend it