Jonah Orion had fallen in battle against the Daemon Prince Kyras, but he had risen in another land.
He had also risen as a pony of all things, one with a horn to boot. Once he regained consciousness, he discovered a village full of what he was now: ponies.
The urge to brutally massacre the lot of them was tempered by age, wisdom, patience, and the fact that horses and ponies were not technically xenos, they had evolved on Earth after all.
These ponies appeared to not have any form of space travel, so he was stuck here.
It's not all bad news though, the warp here is calm, structured, harmonious… the very opposite of its normally chaotic self, he also retained possession of his Force Staff and his armor.
Then he stumbled into the village, aptly named 'Ponyville', and discovered that they had a severe lacking of a proper Librarium.
And there was that monstrously powerful psyker he detected, and was in turn detected by.
He hoped he wouldn't die quite so soon after dying.
That would be tedious.