Lord Cthulhu is the highest level boss before hardcore mode. He is a mercenary class boss that requires a large group to kill. He is located on the planet Ihosota in the Andromeda system.
This boss is very different than most when it comes to fighting as you cannot sit around and just turn as he moves due to the Thumpers(droid like mobs that home in on the player with aggro and explode when close) he summons. A common tactic is to slowly move around the edge of the room letting both the boss and thumpers follow who ever is currently tanking.
When making a group for killing Cthulhu you generally want 7 or more players consisting of but not limited to 3 engineers, 2 dps (pyrokinetic preferred due to the high damage from decimate), telekinetic for augment and debuffs (can also be used to tank with the right setup), telepath for buffs and debuffs. This is by no means the best or only way for a group setup but has a high rate of success.
Due to the shear amount of damage he does to the tank, the tank will likely change often (either due to death or to prevent death). At least 2 of the engineers have to keep a eye on the aggro to make sure to be repairing the correct tank, having a 4th engineer helps with this as it means more consistent repairing and faster the other tank is able to take hits again.
Along with normal boss loot like plates, systems, weapons and conduits Lord Cthulhu also drops high level kits for fabrication, and a small chance to drop an alien with a +8 attribute.