The Keeper is the main antagonist in the second season of Legend of the Seeker. He is "known to be the enemy of everything that lives, walks, and breathes". The Keeper has many servants including Darken Rahl. He uses Banelings as the main soldiers of his army. He has also created the Sisters of the Dark to serve him as well.
At the Beginning
The Keeper was known to have a relationship with the Creator, but he grew jealous of her children and betrayed the Creator, by creating death and releasing it on her children. The Creator wept for days without end, and a Stone was formed from her tears, and so she used its power to seal the Keeper in the Underworld for an eternity.
War on the Land of the Living
When the Veil was torn due to the destruction of the Boxes of Orden, it allowed Keeper of the Underworld to unleash his banelings and other kinds of evil creatures to terrorize and wage war on the land of the living. He usually communicates with his servants most notably the Sisters of the Dark through Darken Rahl his cheif agent. He employs the Sisters of the Dark to kill the Mother Confessor Kahlan Amnell since a prophecy states as long as she lives he is doomed to fail in his plans. At one time the Keeper heard that the Creator was in the Land of the Living in the body of a girl named Maia, he commanded Darken Rahl to open a rift and take Maia, but Richard saved her and Maia used her omnipotent powers to close the rift. Later the Keeper was enraged that his cheif agent Darken Rahl betrayed and so commanded Sister of the Dark Marianna to kill him, but she failed twice. When the Wizard of the First Order Zeddicus Zu'l Zorander accidently warps reality and time, the Veil was repaired and Keeper's war was prevented and so to start the war again the Keeper commands Sister Marianna to collect the Sword of Truth and the Boxes of Orden and destroy it, but she fails again.
Fed up of his servants failures he creates a rift to prevent Richard's friends from going to the Pillars of Creation. He then takes the body of a boy named Declan and manipulates Richard into giving him the Stone of Tears and leading him to the Pillars of Creation when he was blinded by Nicci (who sent shards of glass to his eyes). He also ordered a last group of Sisters of the Dark to kill Kahlan (but they failed as she was in the Blood Rage). Later after taking the Stone of Tears with him to the Underworld, he was sealed once again to the Underworld forever by a new Stone of Tears created from the Mother Confessor Kahlan's tears.
As the Keeper of the Underworld he is a being of immense power. The powers he demonstrated is the ability to make Banelings, people who have taken his twisted deal and ressurect the dead. Since he granted Darken Rahl some of his powers that means he can also open rifts to the Underworld and use the flames of the Underworld to torment many people. He can also take the body of a dead person.