Some TMNT stuff really isn't for little kids.
Loki is the Norse god of lies and treachery. He is often the antagonist in stories involving him, and is the catalyst of the end times in Norse mythology, Ragnarok.
In Halls of Lost Legends, the Ninja Turtles stumble upon a massive tower. Once inside, they run across a young sprite by the name of Locke. He tells the Turtles of the Hall, describing how myths and legends take on an actual existence upon belief, but when that belief does not continue, they are enshrined as statues in this place, with Locke as their curator. Locke says that the Turtles are there much too early, and that they should leave, lest they create great disturbance.
Donatello seeks to investigate a reasonable explanation to this bizarre event, and runs into a room in the Hall labeled "Asgard". There, he sees Odin frozen in place, and realizes upon seeing the god's reflection in a shield (adorned with a turtle symbol), that he is crying.
After saving his brothers from Cronus, Donatello reveals that he has deduced Locke's true identity, and uses the shield to reflect the sprite's true nature as Loki. Loki summons a quartet of beasts - Fideal, Cerberus, a Mantichora, and Grendel. The Turtles makes short work of these monsters, and Michelangelo stops Loki from summoning any more. Leonardo puts his katana to Loki's throat and forces him to release the legends frozen within the Halls.
Merlin, freed from the curse, explains that Loki discovered this curse and froze all of the legends and intended to fill it with the less noble entities of myth, such as Cronus and the monsters he summoned. The legends used their last ounces of willpower to summon the Turtles to help them. Merlin then states that Loki was returned to his prison to await Ragnarok.