https://f95zone.to/threads/naedoko-demons-ground-v1-12-tsukinomizu-project.79560/
Unsure how much you mean with "as a game mechanic" as it is more the birthing (of monsters) that are important tbh, but that ofc by extension means you need to get impregnated somehow. So basically you fight with a team of monsters (a bit like pokemon basically in that regard), but instead of going out and capture monsters, you get impregnated mainly by tentacles, and in a lot of cases by being defeated (with some exceptions strewn around for both of those things), and after that, you go back to base to birth your new monster (they come out of eggs too, so more egg-laying I geuss, that do hatch immediately, you don't really see the hatching, just get the monster right away after the birthing/egg-laying scene), except for a single tier of monsters that come out as small tentacles)
So yeah it have a LOT and LOT of tentacles, and a fair bit of Futanari as well if any of that matters to you.