Board Thread:Dark Souls/@comment-2.50.226.242-20160712195217/@comment-4511886-20160712205524

The door is unlocked with a key found in a chest behind the Serpent Mage.

The Pisacas only come out once. Reloading the area, for example by dying or leaving the area, will render them all stationary within the room until you aggro them yourself. To talk to Logan, kill all of the aggresive ones and you should have no trouble.