Lil' Brimstone gives Isaac a small demon Familiar whose horns are a smaller version of the horns the player gets when he picks up Brimstone. Lil' Brimstone fires a small Brimstone laser. It cannot destroy Fireplace or Poop.
It can be found in the Treasure Room and in the Devil Room where it costs two heart containers, or with Judas' Tongue, 1 heart container. The seed to get Lil' Brimstone in the first treasure room, you can use the seed ABG1 BKJZ, EF4L 842B or 4C87 EHZJ (Seeds only work with Rebirth).