OK, color me dumb. I found out how to view existing synonyms (Tags=>synonyms - https://scifi.stackexchange.com/tags/synonyms ), but how do I create a new one?
I want to synonymize "Horcrux" and "Horcruxes" tags, since there seems to be a constant drift with some question getting both. I tried to merge them into Horcrux 3 times already, and new ones keep popping up.
I don't much care which of the two is the main tag and which is a synonym - merely that there aren't two, EVER again.