While MK was training with Monkey King, Mei called him to tell him that the Smoke Monster was attacking the city. When MK attempt to stop it, the Smoke Monster easily defeats him, but was stopped when Macaque arrives.
After Macaque found out where the Smoke Monster lived, he takes MK there so he can destroy it. When he did, the Smoke Monster steals MK's powers and gives it to Macaque, revealing that it was actually working for him all along. When Macaque used his shadow clones to immobilize Monkey King, he used the Smoke Monster to finish off MK.
However, when MK remember what he learned from his training, he regains his powers and used the Golden Staff to defeat Macaque, destroying the Smoke Monster in the process.
The Smoke Monster was a giant-sized demon made out of purple smoke that resembled a mummy with pink glowing eyes.