The Monkey King is a demon. He was released by Chris and Jenny, saying three words spelled by a ouija board: "Ommox Hodael Kamara". The demon killed them both but needed a master to stay in earth, so he adopts their son, Paul, as his master. He uses fear to feed and to kill and has the ability to change his shape into the worst fear of the person he's attacking, or is seen as such by the creature who sees him. We can see how he adopts many shapes, when seen by Abby, he is seen as a giant spider, when seen by The Swamp Thing he is seen as fire (as in the cover of The Swamp Thing #26), and so on.
When he killed Chris and Jenny, Paul was transported by the government to Elysium Lawns, a center for autistic children in Baton Rouge, Louisiana where he scared the children and feed with their fear gaining power and strength.
The demon Etrigan finds out that he has escaped and goes after him; he and The Swamp Thing, along with Abby Holland, who was then working at Elyrium Lawns are trying to stop him from hurting the children, but it's Paul who defeats him fronting him and his fears, by doing so, the demon gets weaker and weaker and is finally eaten by Etrigan.