Dark Spiderling Invaders are found in Aura Mar, Caldera Mar, Sentinel Point Zeta, and Rivendark Canyon. They have the same attacks as normal Spiderlings, although they deal much more damage. They are also faster and smarter than average Spiderlings, able to turn around quickly, and charge at enemies. They have around 27 health. It states on the achievements that this is a mini-boss.
They are led by Muffet Bane, who spawns to inspire his troops.