'M'

M is the monster seen in the second OAD. The monster's real name is unknown. In the Extra: The Tragedy of M?, it eats the which Shion had made for Rimuru which causes it to become highly aggressive due to the poison.

Appearance
Its true appearance is mostly unknown though it features a very large spiraling cone which has varying sized eyes around the bottom spiral with spikes and coral dotted around its cone. It also has yellow tentacles which it uses to grab any prey or food which happens to fall into or be swimming in the lake.

Trivia

 * The Sarasa sand is made when sand combined with water and 'M's mucus. It is a soft and smooth, bead-like sand perfect for making Rimuru Replica cushions.
 * Upon curing the poison, Rimuru teaches 'M' Magic Sense.