Project Erebos is a failed "Perfect Mutation" of the Zombie Virus. It was in containment near Bio-Dome B, but managed to escape, killing its captors and breaking its way free. Physically, Erebos is large, tall, and humanoid in appearance. Erebos has large fangs with a heavy under-bite, as well as grey skin and 4 long jointed legs. Its back is covered in bulbous pustules and it possesses two clawlike arms.
Erebos is one of the few bosses who's difficulty and skill set changes dramatically across difficulties. The location in which he is fought also changes as a result of common tactics employed in said difficulties.
On the lower difficulties, Erebos will have a highly simplified set of powers. His Primary attack is his melee sweep. It has a very short range and is easy to avoid. With careful kiting, players can easily avoid getting close enough to Erebos to be hit by his attacks. Slowing skills and stuns will work well to incapacitate the boss and make him easier to deal with.
Aside from his melee attack the only other ability Erebos possesses is his Meteor attack. These large spore bombs fall from the sky at random. Their impact locations are highlighted with a red box and players will have several seconds to move before they detonate after impact. The explosion the meteors create does roughly 50 damage and can inflict ailments. Best to avoid these at all costs, they can easily cause low health players to lose a life.
When heavily injured, usually around the 3000 health remaining mark Erebos will flee and the boss battle will be over. Be cautious during the last seconds of the seconds of the fight as he may get a few more free hits in before he burrows.
The best weapons available to deal with Erebos are the Crowbar, Flamethrower MK-3, M5 Pulse Rifle, 590A5 Combat Shotgun, P-45 Gauss Pistol, and M45 Marksman Rifle. The flamethrower will yield the highest damage, but also forces players to engage at the close distance.
On the harder difficulties, Erebos will display his true Power. His melee attack is replaced with a ranged attack with much higher splash damage. It will also bypass standard Armor Mechanics. His Meteors will now spawn creep after detonating, and are even more painful. He will also drop them in larger barrages and more frequently. The extra lethality and decreased player mobility courtesy of the creep make a drawn out battle very dangerous.
A good tank is massively helpful here. On these difficulties tanks can reach their T3 skills by this point, and the venerable Reactive Armor makes Erebos a pushover. Erebos’s best weapon to cause chaos and ruin an uncoordinated team is the addition of "Yellow Erebos." He will briefly flash yellow, and will blind all players for several seconds. Problems typically come from the combination of blindness with Creep, Meteors, and Gargoyles. To avoid this, kill him as fast as possible. The best counter is to have an Engineer's Field Nexus. It is also a good idea for several players carry and use Road Flares. Other sight sharing skills will do the trick as well, especially the Recon's flare.
Erebos will periodically drop Meteors from the sky to crush players. On Recruit mode, these will be outlined by Red Indicator Boxes to warn new players. On higher difficulties this box is replaced by a small blood spatter on target’s feet. As difficulty increases, the frequency and amount of meteors dropped increases. These meteors will produce Creep on impact, making them even more dangerous.
Only seen on harder difficulties such as Nightmare, the pustules on Erebos’s back will turn yellow and he will blind all players for a short duration. Road Flares or a Recon can shed some light to help out, or an Engineer's Field Nexus can keep the team illuminated.
When significantly injured, at roughly 15% health, Erebos will burrow and flee. This marks the end of this stage of the battle.