There are good and bad realms, realms of destruction and realms of creation. One of the most evil realms to exist is…. The Underworld. Home to the skeleton warriors and a shadow version of Lord Garmadon. 

Samukai

Let’s start off with Samukai, the ruler of the skeleton army. According to Ninjago wiki, Samukai died and was reborn as a skeleton in the Underworld. He reunited the undead and formed a skeleton army and became their king. That is, until Garmadon was banished to the Underworld.

Shadow Lord Garmadon

Lord Garmadon (who was a dormant shadow), made Samukai his second in command and his servant along with the skeleton army. Lord Garmadon also forced him to get the Golden Weapons. Since Samukai had four arms, only he could possess all four weapons at once. (Remember that this is before Lord Garmadon grew four arms). Back then, all Lord Garmadon wanted were the Golden Weapons since they would make him powerful enough to rule Ninjago. 

During the show, the ninja had just gotten their golden weapons and their dragons. They then ventured to the Underworld so they could stop Lord Garmadon. In order to get in Garmadon’s “dark fortress”, they first needed to get past the skeleton army and guards. The ninja decided to fight the army using their golden weapons and spinjitzu. Instead, the skeleton army ended up beating them and they needed to think quickly about what to do next. They discussed using the Tornado of Creation to beat the army and then used their combined spinjitzu to perform the Tornado of Creation which won them the battle against the skeleton army. 

Tornado of Creation

In the end, the ninja got to The Dark Fortress and beat Samukai but unfortunately Garmadon escaped through a portal to cause chaos in Ninjago. Since Garmadon also grew his four arms, he could possess The Golden Weapons, only making things worse for the ninja.

The Dark Fortress

 To wrap it up, I think that The Underworld might come back in future seasons since there have already been signs of it in several episodes. For example, in season five of Ninjago, Lloyd used The Realm Crystal to travel through different realms, including The Underworld.