I Became a Leader in a Wretched Prison - Oneshot

[MangaDex]

 

[Hekela]

This was a promo for the novel under the same title (which is honestly fun to read so far, I've read to C.101), but it has since become a proper manhwa, which you can find HERE (<-click for link). Mangadex only has up to C.025 at the moment, but you can find more chapters (up to C.044 currently) on other sites. (I'm not sure I'd be allowed to post links to other manga sites, so you can just google the name to find it)
If you're interested in the novel, you can find it HERE (<- Click for NU page link)