From the Shadows in Ninjago/Lord Vortech released the Overlord and his alliance

Here is how the Overlord and Lord Vortech formed an alliance in The Overlord Returns.

Suddenly at the Shadows of Ninjago, Lord Vortech appears out of nowhere.

Lord Vortech: The time has come for revenge.

Just as he feels the portal of Tartarus, Vortech felt a dark shadow of certain being.

Lord Vortech: After all these years of searching, he really exists... The Overlord.

So, he opens the portal to Tartarus and released him from imprisonment along with his wife, the Preeminent.

The Overlord: Lord Vortech, why are you here?!

Lord Vortech: Just thought you could use my help, I'm the one who freed you from Tartarus.

The Overlord: What do you have in mind?

Lord Vortech: I figured that if we combined our dark purpose, we'll rule the entire LEGO Dimensions, and not just Ninjago or Chima. Once we get our hands on the Foundation Elements, we'll be unstoppable.

The Overlord: Very well, Vortech. I accept your alliance. And in return, you help me capture the alicorn princesses of Equestria.

Lord Vortech: Well, that can be arranged.

Soon enough, all evil beings of the LEGO Dimensions were gathered together for a dark purpose.

Lord Vortech: Friends, it's time we begin our revenge! And once we're through with all who'd dare stand in our way, we shall rule the entire LEGO Dimensions once and for all!

The Overlord: Who will come, who will join us, who wants their revenge against our enemies!?

With that said, all the evil LEGO Villains cheered on Lord Vortech and the Overlord's leadership.