The Season of Mastery event servers for WoW Classic have gotten another update during the beta test, and that means some new spells in the latest build. Now, technically, there’s no certainty about what these new spells are for, with only some of them having effects or descriptions to be mined out. However, it seems pretty likely that these are the first passes on spells to make bosses more difficult in keeping with the game’s last development dispatch about adding new mechanics.
Like, it’s not certain, but it seems fairly probable that “Summon Onyxian Warder” is going to involve adds on the Onyxia fight rather than being a new Paladin ability for this special server. Just a hunch. You can check out the datamined spells and the descriptions available for same right here, although it’s only a peek inside what is happening rather than a clear explanation of any and all new mechanics being introduced through Molten Core and Onyxia’s Lair.