The Assimilated is a massive boss lurking in the Foundry. He is one of the possible bosses for the 3rd Section of the raid.
After Dr. Sheldon Harvey was able to successfully launch his Universal Purpose, he assimilated with a supercomputer to form this entity. Being no more than a giant computer physically, it can not attack you directly, relying solely on a its laser system and robotic drones to protect itself against attackers who seek to destroy Apex's ultimate success.
Should you manage to shut down the Assimilated for good, you will find yourself rewarded with an AI circuit and access to the final wing of the Foundry.