Down the ladder is the shrine room where the Shira summon is.
Break the illusory wall and head down the path.
Look for the statue not holding a sword, it is an illusion.
Head left at the junction into a circular room filled with statues.
Using the lift, Halfway down there is an opening, jump/roll in.
Darkeater Midir is found in The Ringed City: After opening the final shortcut gate near the Ringed Inner Wall bonfire.
Midir, descendant of the Archdragons, was raised by the gods, and owing to his immortality was given a duty to eternally battle the dark, a duty that he would never forget, even after the gods perished. Darkeater Midir is a Boss in Dark Souls 3, available with the The Ringed City DLC.