You need to be at least Level 50 to receive this quest.
You need to be at least Level 44 to turn-in this quest.
Go with Namidas to the cave to explore and cleanse.
Hail Namidas to begin following him. When you enter the tunnel four Bloodstained Tar Beasts appear and attack. Namidas will try to cleanse them but he will fail. You will have to kill them. Meet Namidas in Hero's Tomb, back where you started, after you defeat them.