This is found in the chest that spawns when you kill the four mobs mentioned in those posts. This is a raid encounter, I'm not sure exactly what level ranges it is intended for, but it should be doable with 12-18 folks of the right mix/gear in era, not much harder than goats in qvic but you need to keep aggro on four things.
Mentioned in a post dating to 2013 on this item by Jyve is further confirmation.
Far end of Thundercrest, heading up to the area that's used for the Yah'Lir event. WAY up NE that you get to passing through center building and onwards.
4 mini-nameds at second to last room. Kill all 4, chest pops with 2 loots that appears to be the same as the Yah'Lir event + this drum.