Almost gonna guarantee it's a rare drop from Alexander himself in A12N and Savage. There's no info about it on the Lodestone, which means its hidden behind their 'spoiler wall' (they don't have Savage drops up on the lodestone, for example, because of spoilers.)
I haven't done A12 enough to see it drop, but there are plenty up on the market boards and enough have been sold to make that a pretty good guess.