At an unknown point in time, the Underworld came into existence as one of the Sixteen Realms, becoming intertwined with Ninjago, with fallen warriors who were unable to come to terms with their deaths arriving in the Underworld and becoming skeletal beings known as Skulkin. Over time, the Underworld's population grew in size as battles continued to occur and warriors continued to fall, with deceased animals becoming enlarged versions of their remains upon their arrival. At some point, Samukai arrived in the Underworld following his death and became the leader of the Skulkin. Later, Samukai came into conflict with the sons of the First Spinjitzu Master and was defeated.