It is located in the Undead Settlements sewers next to the Dliapidated Bridge after purchasing the grave key.
If you kill Irina of Carim at Firelink Shrine he will show up there when you return, hostile.After freeing Irina of Carim from her cell in Undead Settlement, Eygon will become allied with you.Eygon of Carim is an NPC in Dark Souls 3.
