Delightful Reaper

The Delightful Reaper is a mutant Heartless created by the Villain League from the souls of Billy and the Delightful Children from Down the Lane. It was a being created when Billy walked into a Delightfulization Chamber while wearing his dad's lucky pants and carrying Grim's scythe in their back pocket. The effects of the chamber reacted with these supernatural items to create the Delightful Reaper. A giant Grim Reaper-like figure wearing the lucky pants and featuring skeletal versions of the faces of everyone it had assimilated. Which at the time of its creation consisted only of Billy and the Delightful Children From Down The Lane, but soon started to grow as the Delightful Reaper began assimilating other children.

It's abilties were being able to unleash waves of energy to assimilate kids into it and being able to cut anything it struck. The lucky pants also made it virtually invicible to some of the most powerful attack sources on earth but only if the attacks were focused anywhere. On the lucky pants which covered the reaper's weak point, it's bottom, even if the attacks did not hit the lucky pants. The reaper would not even take a significant amount of damage to even being wounded at all (Probabely this is because the reaper could only be destroyed if the attack destroyed it's weak point, which was armored by the lucky pants.).

It's other ablity was to create massive storms and dark power at will but this ablity was only briefly seen. When Mandy took control of it before Billy's dad pulled down the pants to reveal the reaper's weak point. It's power was so great that a single clash with an equally powerful force would force back it's opponents but leave the reaper unfazed, finally it was quite fast despite it's size being able to use the lucky pants by turning around to deflect an extremly fast attack from Mandy's M.A.N.D.R.O.B.O.T..

Soon, it had assimilated nearly everyone on the planet, whose skeletal faces at this point formed into a shape resembling Grim's face. Numbuh 1 and Grim, who had fused together to form a giant skeletal samurai monster using the "Bone of Barnacles", then arrive to fight the Delightful Reaper, but are unable to defeat it due to the lucky pant's immunity to lasers, supernatural energies and mustard.

As it is about to assimilate them, Mandy's M.A.N.D.R.O.B.O.T. arrives on the scene and puts on a decent fight, but the reaper is not even injured, the only "wound" being a quick-fading mustard stain on it's robe, but instead of pulling down the Reaper's pants, which is the only way to defeat it, Mandy allows herself to be assimilated by the Delightful Reaper, becoming its dominant personality and granting her unimaginable power.

Billy's dad then appears, demanding the return of his lucky pants, and leaps to the height of the giant monster's waistline, pulling off its pants and rendering it vulnerable. The Delightful Reaper is then destroyed by Grim and Numbuh 1 hitting it's weak point and releasing the children it assimilated. As the Delightful Reaper explodes, several characters from other Cartoon Network shows are seen flying out of it.

Role in the series
After the Delightful Reaper was destroyed, Malefor used his dark magic to extract the creature's soul and make it a single being from the rest of the souls it had assimilated, and turned it into a Heartless for Mirage to control.