Mob spawned as Coal Golem at -694, -2940, 145, regular Thundercrest Isles zone-wide golem model, but very small. Conned green to 91, and must have something to do with a Frostfell quest due to the item he dropped and his aggro message, "I've got something for your stocking!"
Coal Golem cast Aura of Ice, spawned six "a frostclaw whirlwind" (dervish model MOBs) and transformed into "Coaleye, Frostclaw Thief" (goblin model MOB). Each mob had approximately 50k HPs. When he transforms and the adds begin to spawn, you see the emote:
Coaleye, Frostclaw Thief shakes violently, dislodging a shocking volume of coal dust and wooden parts. "Well let's get on with this! You obviously know who I am, so there's no need for the disguise. It's time to show you what really happens when you're naughty!"
It also begins to snow.
Sorry to put all of this here, but I couldn't find Coal Golem, Coaleye, Frostclaw Thief or a frostclaw whirlwind in the database. Feel free to move the post to a better spot if you can find one.