Mother Long Legs, also known as Terror Long Legs and Grandpa Long Legs, are a new creature added in the Downpour DLC. They are a larger version of the Daddy Long Legs and are the biggest mobile Rot cyst.
Mother Long Legs are large, black, bulbous masses accented with violet X shapes, with several violet-tipped tentacles extending from their core. The shape of their core and number of tentacles they have may vary.
Fighting a Mother Long Legs is an exercise in futility. Despite having health, they're nigh-invulnerable to almost all weapons and take almost no damage from them, not even from things that are usually deadly to Rot cysts like an Explosive Spear hitting their cores. The only thing that can kill a Mother Long Legs efficiently is a Singularity Bomb, as they can't resist its instakill effect. Short of this, killing one in the span of a normal cycle is impossible. Further frustrating this issue, their legs seem to take incredible amounts of damage to permanently disable, if they can be disabled at all.
Like the other Rot-based enemies, Mother Long Legs are blind. However, they have an acute sense of hearing that allows for them to pursue the Slugcat and other prey. Distracting one with thrown Rubbish is the easiest way to evade it.
Mother Long Legs exhibits the same hearing ability as the Daddy Long Legs, actively searching for prey to consume either by inserting their tentacles into small crevices like pipes and tunnels to grasp hiding prey or approaching the source of loud noises. When a creature manages to evade its tendrils, the Mother Long Legs lingers for a while in the same position where the creature escaped, attempting to fit into and use its lengthy tentacles to grab and pull it out.
They devolved in the Garbage Wastes, becoming Daddy Long Legs and eventually Brother Long Legs over time. This implies that the Rot is likely incapable of surviving—or at least spreading and evolving— outside of Five Pebbles.
Interestingly, Mother Long Legs are able to consume other mobile Rot cysts should they come in contact with each other, including Daddy Long Legs.
Mother Long Legs attempt to eat each other, but are unable to.