Overtakers are what the Imagineers call the Disney Villians because they try to take over the parks, making thier own version of fun. Finn and the Keepers have to fight the villians to take back the parks and keep the magic. But you must know there are 2 kinds of magic. One good magic used in the parks now, and a black magic the Overtakers use to fight the Keepers to gain the parks.
Originally, the Overtakers were disorganised and fought among themselves during the time the park was closed. However, Chernabog united all the villains under his and Maleficent's control. Unfortunately, the Imagineers put Chernabog on ice, literally, by trapping him in Expedition Everest as a Yeti. This left Maleficent to try rescuing her master for several years. To combat the Overtakers, the Kingdom Keepers were founded, and mentored by Wayne Kresky.
Following Dark Passage, the Overtakers have taken severe causualties: Chernabog was trapped in a flooding maze, the Queen is lost, and Maleficent's butt was ripped through by Finn in DHI form. However, two powerful spellcasters remain on the Disney Dream: Tia Dalma and Jafar.
O.T.Ks or Overtaker Kids are kid who joined the overtakers and are DHIs. Many of them are jealous of the Keepers, or have a grudge.