Tentacle monster.

A subterranean temple beneath Manhattan appears in the 1987 cartoon episode Splinter No More.

Back in the 1920's, a cult used the temple to open portals to other dimensions by saying a spell. When Shredder learns of it, he obtains the spell in order to reach Dimension X. However, Splinter's intervention accidentally opens a portal to a different dimension, causing an extraterrestrial tentacle monster to appear on Earth. To get rid of the monster, Shredder has to team up with the Ninja Turtles and trick the monster into destroying the entire temple.

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