The Obeahman is one of the most powerful ghosts in the world, next to Anna. He is brutal and is the cause of Cas' father's death. He is a master of West Indian voodoo and used it to curse Cas, giving him until morning to kill Anna, or else he will die of poisoning.
Unbeknownst to Cas, every time he destroys a ghost, the power from that ghost gets fed into the Obeahman, ultimately making him more powerful.
The Obeahman has a direct link to athame, making him unable to be destroyed using the knife.
This eventually becomes his downfall when Anna sacrifices herself to destroy the Obeahman, opening a portal to Hell and dragging him inside.