Defeat the mutated Moreau.
Story related, can’t be missed
After opening the dam to drain the reservoir, you will follow the mutated form toward the clinic. The first phase of the fight is straight forward, just keep shooting the “tongue” with the sniper or Shotgun. The next phase had a few breakable barriers put around the area, with a new acid spit attack to go along with the standard charge and bit. Just lead it around the area and shoot the explosive Barrels when close to save ammo. Using all Barrels will kill it on casual.