I've seen this in multiple combinations, but the one I was specifically looking at was:
「背負いし者」
...from 「宿命を背負いし者」.
Similarly,
「かつて来たりし者」
Scouring through all my grammar books, I can't find this form of し explained. A Dictionary of Basic Japanese Grammar (Seiichi Makino & Michio Tsutsui) lists し as a particle, roughly translated as 'and', or as an infix attached to an い-adj. In this context, the bit being modified by し is a verb, so would the infix work the same way? I doubt し is a shorthand for する, given that it's directly modifying the noun after it. Granted, I'm a beginner in Japanese.
