Lord Vortech regained his strength/Freeing himself, the Overlord and the Preeminent

Here is how the Overlord returns for revenge in The Overlord's Revenge.

But not too far from Ponyville, Lord Vortech regained enough strength.

Lord Vortech: At last, my strength has come back to me!

As he opened the portal to Tartarus, the Overlord and the Preeminent joined him.

The Overlord: The time has come for our revenge, Lord Vortech!

Lord Vortech: Yes, Overlord. So, I take you have a plan of your own?

The Overlord: As a matter of fact I do, Vortech.

So, the Overlord raised up his Stone Army as Lord Vortech gathered the Nindroid Army.

The Overlord: It's time we bring out the Realm Crystal.

Lord Vortech: That's it, why can't we just use the Foundation Elements while we're at it?

The Overlord: Because they're powerful enough for the Dragon Warrior and his allies to defeat us?

Lord Vortech: I see what you mean, Overlord.

The Overlord: Soon, they will pay for their interference.

Lord Vortech: And so will those Master Builders who defeated us along with the chosen heroes.

The Overlord: The Realm Crystal is the only weapon that will help us control space and time.

Lord Vortech: Now, this should be more interesting as I thought.

The Overlord: Revenge is mine.

Lord Vortech: Don't you mean ours?

The Overlord: Of course. Here. I want to give you something. (giving him the Dark Realm Crystal) This is your new Foundational Element, the Dark Realm Crystal.

So, they gathered their armies and set out to work their dark purpose for revenge.