Wrathstone

Source
This item can be found inside Cache of Living Stone after defeating Kologarn in the 25-man version of Ulduar.

Effective Attack Power
If activated on every cooldown, this trinket provides an average of: 856 * 20 / 120 = 856 * 1 / 6 = 856 / 6 = 142+ 2 ⁄ 3 Attack Power