Menderbug

Behaviour and Tactics
Menderbug has a 2% chance to spawn at the entrance of Forgotten Crossroads right below Dirtmouth. To spawn, the following conditions must be met:
 * The sign at the entrance must be destroyed
 * The player must leave and re-enter the room

Upon seeing the Knight, the Menderbug will fly away and quickly leave the room.

To kill Menderbug, enter the Forgotten Crossroads from Dirtmouth using Desolate Dive or Descending Dark. This will destroy the sign upon landing and, if they have spawned, will also kill the Menderbug. They can also be spawned by leaving the room to the west or east within Forgotten Crossroads, however, this method takes significantly more time and makes it easier to miss the kill. One of the easiest ways to catch them is in late game after acquiring Kingsoul and/or Salubra's Blessing to refill SOUL easily.

Menderbug's Journal
After killing Menderbug, their house will be unlocked. The house can be found in the same village where Sly is first found infected.

Trivia

 * There is an unused key found in the game files named Mender_Key and in early dev notes it is implied that Menderbug would drop this key called an "Odd Key" that would probably then allow access into his house.
 * Killing Menderbug is not needed in order to acquire the Hunter's Mark.
 * Menderbug cannot be read by the Dream Nail.
 * Upon death, Menderbug emits an Infection cloud, despite not showing any hostility like infected creatures do.