Haunted Roof Inner Corners were implemented during the May 2017 launch update, together with the other Roof and Slope Corner blocks.
The Haunted Roof recipe drops from ghost mobs during the Haunted Nights event. It can also be found in various adventures created by other players.
Inner Corners are created by putting Haunted Roof blocks into the processor, giving you 1 inner corner block per roof block.
The roof blocks can be rotated by pressing and holding R and moving the mouse while holding the left mouse button.