LeBron's Disturbing Pennywise Costume Highlights Cavs Halloween Party

LeBron's a nightmare to opponents on the court ... but he was even more frightening at the Cavs Halloween party, 'cause the King's Pennywise costume is sure to keep you up at night.

The Cavs always go over-the-top for their annual team gathering ... and Monday night was no exception -- from Kyle Korver's Willy Wonka (complete with Oompa Loompas!!) to J.R. Smith and his wife, Jewel, going as the Coneheads.

But it was Tristan Thompson and Khloe Kardashian taking the best couples costume award for their Khal Drogo and Daenerys Targaryen outfits from "Game of Thrones."

Check it out ... the party was lit.