OMEGA
The Oni warlord brushed a fragment of charred armor off his own as the portal closed behind him. He had teleported alone; without the Realm Crystal, he was unable to bring all the Oni at once.
That was fine with the Omega. He wanted to do the job himself. The part-Oni was extremely aggravating. He tried to become an Oni, thinking he was like them. But he wasn't.
The Omega let the others know that the first son of the hybrid was dead. The second son, however, was protected by his color-coded... ninja.
The language that once was familiar to the Omega had been twisted and mangled in the hands of humans. He was barely able to understand and speak to them, but he was an Oni of few words. There was no need for long monologues as the opponent dies. Get the job done and get out of there, except when you're taking over a realm.
It didn't take a genius for the Omega to realize the second son- Wu, was his name?- wasn't afraid to lay down his life to protect others, similar to those ninja. But he had trained them, raised them, taught them the ways of Creation, and element the Omega had unadulterated loathing for. Wu was the one at fault.
No, the best way to teach Wu to never mess with an Oni wouldn't be to kill him. The first son, Garmadon... the Omega didn't care for. His ego was too big to allow the Omega to take over Ninjago, so he paid with his life. The Omega rarely engaged in a good sword fight, so he enjoyed their battle thoroughly. Garmadon had put up a good fight, but not good enough.
But Wu, on the other hand, didn't care about how much power the Omega had; he cared about the survival of others, especially those close to him. He was the one the Oni really wanted to get back at. He and his rainbow students had re-banished the Oni to the Realm, but it was rather pointless as Oni could teleport-- at least the Omega could, and he was the most dangerous and important one.
But the Oni knew he couldn't fight all seven at once, plus their comrades. The best way to make Wu pay for this was to get help from those ninja's worst enemies. He had tapped into several of their minds during the fight. He knew the first person he wanted to bring back-- and it involved a teapot.
He wanted to see Wu suffer. The Omega wanted the second son of the hybrid to see what an Oni could really do.