at the end of mk5 before Earth realm warroirs got to them:-

Shang Stung and Quan Chi argued and battled for the amulet between themselfs in the lava shrine. during the fight reptile appered but was not noticed by Shang Stung or Quan Chi. the egg in the lava shrine cracked open and reptile transformed into the Dragon King. Shang Stung and Quan Chi stood in shook. the Dragon king seeing the amulet battled the two sorcers for it in a three way battle. the dragon king won and made Shang Stung and Quan Chi his slaves, along with his almost complete undead army.
but for Earth realm, there was some hope as Liu Kang had been receracted by the elder Gods as Liu Kang was the champion of Mortal Kombat and had been killed in a false match, breaking the rules of Mortal Kombat.

p.s Shinnok could be in the game as the amulet used to belong to him for thousands of years so he must have something to do with the Dragon King.