Haunted Stairs Outer Corners were implemented during the May 2017 launch update, together with the other Roof and Slope Corner blocks.
Haunted Stairs Corners are created by putting haunted stairs blocks into the processor.
The Haunted Roof recipe drops from ghost mobs during the Haunted Nights Halloween event. It can also be found in various adventures created by other players.
The stairs corner blocks can be rotated by pressing and holding R and moving the mouse while holding the left mouse button.