it-belongs-in-a-ruin should be sufficient, hoping people will simply autocomplete the tag.
belongs-in-ancient-ruin is one choice, keeping all the "content" words. It should be clear. The problem is that if someone types in "it-" it won't pop up as a suggestion.
it-belongs-in-a-ruin is fairly clear and will pop up as a suggestion if you type from the beginning. It does, however, eliminate the suggestion for when someone types "ancient".
it-belongs-in-an-ancient is similar. It's not a complete sentence, but I'm not sure that matters -- the truncation seems noticeable, which would lead the tagger to (rightly) assume that it's the game they're after.
I'm assuming ibiaar is not a well-known acronym.
I think you're kinda screwed no matter what; I would go with the third option (it-belongs-in-an-ancient), myself. It only really hurts super-easily-confused people and fast typers who get in "ancient-ruins" before looking at the suggestions.
You could possibly have tag synonyms for some of these to help with the pop-up suggestions, eliminating any missed cases.