Ravenous snail

The Ravenous snail appears near Fairy ring BKR in the Mort Myre Swamp when the player is on a Slayer challenge assigned by Mazchna or Achtryn. To activate the challenge, the player be near the ring and must contact Mazchna or Achtryn using an Enchanted gem. The monster will then charge out of the swamp toward the ring and attack the player.

Other drops
Always drops Thin snail (but no snail shell, unlike other snails); unknown if it ever drops anything else (no reports of other drops yet).