Kraang

Kraang are alien robots that were sent to earth so they can take over it. They were first appeared in episode Rise of the Turtles when they kidnapped April and her dad. They are not actually robots, but small pink brains that enter into robots so they can attack, speak, etc.

Ooze
Kraang have a secret weapon called "Ooze". With the ooze, Kraang mutated splinter and the turtles. they also used it to mutate other people like Snakeweed, Spider Bytez, Dog Pound, Fish Face, etc. They also mutated some of the turtles allies like Slash, Doctor Rocwell, Leatherhead, etc. After the invasion on New York, the Kraang used ooze to mutate the humans.

Army and Weapons
The Kraang have an unlimited army of robots and lots of weapons and machines. The robots use a weapon that shoots lasers to attack the turtles. Kraang also created lots of machines, so they can beat the turtles and take over the world.

The Invasion
Kraang were planning their invasion on New York for a long time. They finally started their invasion on episode "The Invasion (Part 1-2) when the co-leader Kraang discovered the turtle's hideout. Leader kraang came and mutanted the humans and then the kraang took them to dimension-X to work as slaves. In episode "Battle for New York (Part 1-2), the turtles retook New York, remutated the humans and sent kraang back to Dimension-X

Trivia

 * Kraang allied with Shredder.
 * Leader Kraang has bigger size than normal Kraang and co-leader Kraang has a robotic eye.
 * Co-leader Kraang discovered the turtles's hideout when April brought Irma there. Irma was not a human but a Kraang with the co-leader inside.