Shadowmaw spawns in a natural cave in the north of the Phocian Swamps. Gives an achievement if defeated in normal mode before leaving Greece. Can spawn 3 adds, shadowlike creatures with high level, and has a fire breath attack that reaches quite far. Wouldn't recommend to melee on a normal playthrough the first time you are at the swamps!
Recommended strategy: Equip a range weapon and hit one time then run back until it attacks, there's a small time where he does the attack animation and should give you enough time to escape, it won't chase you to the entrance of the cave so it's recommended to repeat the strategy in the entrance of the cave. It takes time but it's the safest way to kill it if you are not strong enough.
Shadowmaw also drops Demon's Blood and Lupine Claw charms often.