After Samael has been defeated, Fast Travel to reach the Crowfather at the Tree of Death. After speaking with the Crowfather, Death will use the two keys and open up the door ahead. Run forth to take on the Corruption head on. Enter the Well of Souls and take on the Avatar of Chaos.
The Avatar of Chaos will swipe at your with its massive blade. Try to avoid these damaging attacks. When you are a distance away, the Avatar of Chaos will jump towards you and try to deliver a blow. When it does, avoid the attack and the avatars blade gets stuck in the ground momentarily. Deliver a few slashes with your own blade.
The Avatar of Chaos will also attack Death with a circular blade attack. This has quite the range so be sure to evade the attack by dashing away from the enemy. During this phase, Death can also summon ghouls to weaken the Avatar. Once its health is low enough, he will fall the floor, and Death can press the action button. The Avatar avoids the final blow and pushes Death away.
This triggers the second phase of the battle. The Avatar now has tentacles of corruption that it will use to attack Death. Other than that, the battle is virtually the same as the first phase. Deliver your scythe attacks and summon ghouls to wear down your foe. After some damage has been done, the Avatar will fall to the floor. Death can press the action button, and does some damage, but doesn’t quite finish off the battle.
During the final phase of the battle, the Avatar has a series of new attacks. The Avatar is more aggressive and will now run towards Death and deliver a series of attacks. Furthermore, the Avatar will slam to the ground, causing corruption to grab Death from below. Jump away to avoid this attack, but it grabs you, press the action button to break free.
The Avatar will plunge its sword into the ground, sending out waves of corruption towards Death. This attack can be easily avoided by just dodging in between the waves. Continue attacking the Avatar with your attacks and summon ghouls whenever possible. Wear the Avatar down until all his health has been drained and Death will summon his Reaper Form to deliver the final strikes.