It's a special hidden monster in the lair of Litvart, related to the Moonlight Sculptor hidden class.
In the novel, it's described as a giant poisonous worm, a snake like monster. In the Manhwa, it looks more like a Scarab type of monster.
During his quest, Weed had to kill 100 of them and it took him 7 days (game time) to achieve his goal. The monster could use thick, green fumes of poison which can eat away an armor and a person.