All credit to Blizzard Entertainment!
No, don't try to find it in WoW's icon lists.
This icon... I think I nicked it from one of websites trying to post stuff around NDA back in early days of WotLK Alpha - I think it came from some closed doors demo playtest or something.
Never found it anywhere else - even reverse image search showed nothing, but as far as I remember, it was one of the first bits of WotLK that came out after reveal. Still don't know why it never made into any release version of the game.
Lost it for years, finally found it yesterday on a forgotten flash drive. Could've kept it as my personal treasure, but eh, at least here it won't be lost again.
Don't even remember original name, pretty sure it was "dark power" something.