Some TMNT stuff really isn't for little kids.
The Foot Clan is a powerful ninja clan in the Batman/Teenage Mutant Ninja Turtles comic miniseries and its sequel Batman/Teenage Mutant Ninja Turtles II. They normally reside in the New York of the Ninja Turtles' reality, and wield a great deal of power over the underworld there. The Shredder is their clan leader and master, and ninja who follow him give up individual names and identities when they become part of the Foot.
When Shredder was transported by Krang to a different dimension, a large number of Foot ninja were transported with him. They assisted their master in amassing power in Gotham City, kidnapping Dr. Naveen Khan and stealing equipment that would allow Shredder to travel between their two worlds. However, their plans were disrupted by the arrival of Batman and the Ninja Turtles, and many of the Foot ninja were beaten and taken captive by the police. They were then sent to Arkham Asylum, where they remained until Shredder was defeated by his enemies, at which time they were sent back through a dimensional gateway.
Upon Shredder's imprisonment, the Foot Clan split into three factions: one third following the Foot Elite, one following Karai, and one following Bebop and Rocksteady. These factions fought against one another, causing a great deal of mayhem, though all of them were eager to kill the Turtles for their part in Shredder's imprisonment.
However, matters became worse when Bane was accidentally transported to their dimension by Donatello. Bane immediately took over the faction ruled by the Foot Elite, and soon forced Bebop and Rocksteady to serve him as well. Only Karai and a small band of loyal followers held out against him. The Foot Clan under Bane began a rule of terror that threatened to destroy the entire city, which was enhanced when Bane began manufacturing the substance known as Venom.
The Foot's loyalties shifted upon the unexpected arrival of their leader, the Shredder, who turned them against Bane and unleashed a massive battle upon Liberty Island. This coincided with the loss of their Venom, which Donatello purged from the Foot ninjas' systems.