Does it exist? Is it even possible? What I mean is this: Essentially, a mod that makes Ruin monsters capturable, and possible re-introduces the Raw Tentacle drop and tentacle biome, complete with the new monsters to populate it. I'd love to have one as a pet on a cultist character, they're very hard to weaken and distract so I feel you should be rewarded for the effort of not killing them and holding back the hordes. An evil cultist lair or a research laboratory in the middle of the biome would be pretty darn awesome. Yes, I'm the type of person who would keep samples of the Ruin despite being hired to slay it. I mean, I already have a cache of tentacle blocks and slime so I don't think a monster would be any "worse". Until we get more biomes and items, I'd love to have some fun with these.
Definitely sounds like a good idea, though it may be a good idea to get well acquainted with everything in modding before actually starting. Sounds like a lot of patching things back in, really.