Mariah Carey Parties with Nick Cannon AND New BF ... It's Magic!!

Mariah Carey can still cast one helluva spell -- she managed to spend Halloween her ex-hubby Nick Cannon and also her billionaire BF, James Packer.

Mimi slipped into a sexy witch costume for her party Saturday night at her rented Airbnb mansion in Bev Hills, but before the shindig ... she and Nick took their kids trick or treating. Naturally, they did it in the most Mariah way possible -- piling into a golf cart with Moroccan and Monroe.

Nick stuck around for the start of the party, but dipped out before Packer got there.

Don't mess with a witch on Halloween ... she'll always get her way!