Knock Knock Knock on Wood, is the 25man version of Freyas Hard Mode and is required to complete the "Glory of the Ulduar Raider" meta achievement and earn a flying mount reward. This achievement is entirely similar when comparing it to Sartharions hard mode of him and the 3 twilight drake bosses.

This achievement requires you to engage and defeat Freya without killing her 3 Elders before battling her, the achievement is also required to complete an objective in the quest line to unlock the Algalon encounter.