The Leghugger is a powerskill that summons a pet Leghugger that follows you and attacks enemies automatically. It has a unique ability to "evolve" after some use, which carries forward to other instances of the same ability within a single run. It is exclusive to the Queen and the Sea DLC.
Leghugger attacks by biting enemies. It deals critical hits on bleeding enemies.
After 100 bites, Leghugger will evolve into adult form.
Activates the skill again to make the Leghugger inflicts bleed debuff to enemies around it. The pet can inflict 1 bleed stack in baby form and 3 bleed stacks in adult form. The debuff lasts for 5 seconds.
After finding the Fisherman's letter in Prisoners' Quarters, go to Toxic Sewers and look for the Fisherman. He will instruct the player to go to Stilt Village. There, a lore room can be found containing an Elite Armored Shrimp. Once killed, Michel's corpse will drop the Crowned Key, as well as a Leghugger, which forces itself into one of the player's skill slots, causing the previous item to be dropped on the ground. The Leghugger is automatically unlocked with no blueprint or cells required once it's obtained.
Synergies
Other sources of bleeding (eg. Blood Sword, Throwing Knife) will satisfy the Leghugger's critical condition when the Leghugger itself is on cooldown.
Crowned Key, as well as a Leghugger, which forces itself into one of the player's skill slots, causing the previous item to be dropped on the ground. The Leghugger is automatically unlocked with no blueprint or cells required once it's obtained.
Blood Sword, 
Throwing Knife) will satisfy the Leghugger's critical condition when the Leghugger itself is on cooldown.
Sadist's Stiletto and 
HemorrhageRotG.
Hand HookTQatS, as enemies thrown by the hand hook that collide with pets are dealt critical damage.
Instinct of the Master of Arms.
Baby Leghugger
Evolved Leghugger
5 