The Abberant Growth can be found near the bat puzzle in Stillmoor (puzzle it as 1705, 2310). It has near 240k health and hits for upwards of 4k on geared tanks. This is a raid monster so bring 10+ friends or a group of five insanely geared players. Loot Darkblade's Notes and bring them to the Faceless Man.
To see the whole quest series at a glance, see The Saga of the Endless Guide .