Speak with Warden Troweller at Kelari Refuge in Freemarch. She tells you to go to Stonefield near Granite Falls to find Nestor. Nestor's body can be between Granite Falls and the graveyard there at 5011, 4853. After you loot his corpse the Bones of Nestor will attack. It has a huge health pool and hits very hard, bring friends.
To see the whole quest series at a glance, see The Saga of the Endless Guide .