Only one monster in the base Catacombs game can perform the transform shot: the Fire Spirit.
If the Overseer hits a hero with this shot, the Overseer has the option to flip the Fire Spirit piece, transforming it into a fire wall. The Fire Spirit remains a fire wall for the remainder of the room and is treated like an obstacle. As such, fire walls can no longer be controlled by the Overseer and do not count as monsters in the room.
A fire wall inflicts one health point of damage to any hero who contacts it, directly or indirectly.
If a fire wall is pushed off the playmat, return it to the point where it departed as normal. If it is moved by another piece but stays on the playmat, it remains in the new position.