The Grim Adventures of the Kids Next Door (2007)

A crossover with The Grim Adventures of Billy and Mandy and Codename: Kids Next Door. While Billy's Dad is underwear shopping, Billy wears his dad's lucky pants, and accidentely rips them with Grim's scythe. He calls Ed, Edd n Eddy, and they recommend Kids Next Door for charity. Numbuh 1 disguises as Billy while Billy is taken to the Deep Sea Lab. Mandy arrives at Billy's house and knows Numbuh 1 is not Billy, and tortures him into revealing the truth. Meanwhile, the Delightful Children From Down the Lane and Billy accidentally fuse with the scythes power to create the Delightful Reaper, and Mandy takes over the KND by fooling Numbuh 362 and the rest of the operatives. Now its up to Numbuh 1 and the Grim Reaper to save KND and stop Mandy and the Delightful Reaper.

Unfortunately the movie The Grim Adventures of the Kids Next Door is not yet available on HBO Max.