The Dark Medusa is basically I, Medusa but with pale gray head and upper body, and magenta lower body. Also, in addition to the Mii eyes on the snake "hair" and a Mii mouth on this Medusa's face, a Mii nose is also attached above the mouth part.
Which Travelers' Hub quests that feature this monster seem to be complete random, so do the dungeon location and the quest rewards.
Chance to use
Inflicts physical damage to one party member.
Turns one party member into stone, preventing him/her from taking any action until freed.