At the bottom of the Tower of Judgment you'll come across Cerberus ~ Warden of Hades (pictured below). This is the boss fight of the tower and can be quite difficult if you lack the appropriate Poison Resistance.
As you can tell from the screenshot above (and my previous paragraph) you'll need some good Poison Resistance for Cerberus. The only other damage that Cerberus does to you is Physical throughout the whole fight. Out of all Cerberus' attacks the two most deadly are Poison Spit and Poison Eruption. Eruption is what you see happening in the screenshot above where Poison explodes out of the cracks in the room.
Poison Spit is where Cerberus shoots a glob of spit at you that creates a little poison puddle on the ground. Even with max Poison Resistance this hurts a lot.
Poison Bite: A melee attack that deals heavy Poison damage too.
Poison Breath: Your standard breath attack, deals heavy Poison damage.
Poison Spit: Cerberus will hock a loogie at you and create a puddle of poison on the ground that you do not want to stand in. Deals heavy Poison damage to anyone who stands in it.
Poison Eruption: All of the cracks throughout Cerberus' room will erupt and deal heavy Poison damage to anyone hit by it.
With Cerberus defeated you'll be able to go into the adjacent room where you'll find the Judges of the tower. Speak with the middle judge to complete this quest and open up the portal to Elysium. Going through the portal will begin the next story quest of Act 4, The Battle for Elysium.