WARNING: The following contains major spoilers for the latest episode of Doctor Who, “The Timeless Children,” which premiered Sunday on BBC.

In the penultimate episode of Season 12 of Doctor Who, the Doctor and her three companions traveled to the future during the final days of the Cyber Wars between humanity and the Cybermen. There, the quartet teamed up with a bunch of survivors, and they attempted to stop the Lone Cyberman from using the Cyberium to return the nearly destroyed Cyber Empire to its former glory. During the confrontation, we witnessed the debut of two new types of Cybermen: the Cyber Drones, as well as the all-new Cyber Warriors.

Both proved to be deadly and frightening new additions to the Cybermen ranks. But it turns out that's not all. In the Season 12 finale, Doctor Who introduces another new type of Cybermen that dwarf the other two in comparison. In fact, these new Cybermen are, easily, the deadliest in the BBC series' history: Meet the Cyber Masters.

Related: Doctor Who Season 12 Finale Leaves the Doctor in [SPOILER]

In "The Timeless Children," the Master extends an invitation to the Lone Cyberman to Gallifrey. There, in the ruins of the fallen Time Lords, the robotic villain arrives aboard his Cyber Carrier, which houses an army's worth of Cyber Warriors. However, the Master quickly turns on his new ally. Before long, the Doctor's nemesis miniaturizes the Lone Cybermen and fuses with the Cyberium. Then, he proceeds to create a new army -- one that will become undefeatable.

In Season 12's "Spyfall, Pt. 2," we learned that the Master had killed all of the Time Lords. However, in "The Timeless Children," the villain reveals that, although he killed his brethren, he actually kept their bodies. He didn't even know why himself at the time, but now, fused with the Cyberium, he gets a hellish idea. The bodies of the deceased Time Lords are used to create a new breed of Cybermen who are appropriately called the Cyber Masters.

Cyber Masters on Doctor Who

These new Cybermen bear the similar build and design of the Cyber Warriors, but they also have the distinct features of the classic Time Lord garb, from the robes to the headdress. What's more, their metallic shells are now inscribed with Gallifreyan symbols. But what is perhaps most noteworthy for the Cyber Masters is that, if they die, they can simply regenerate back to life.

Therefore, this new army -- the Master's new army -- simply can't be killed. This easily makes them the most dangerous Cybermen of all time and makes them potentially more of a threat than the Daleks.

By the end of the episode, Gallifrey is destroyed, but the Master still manages to escape with a handful of his Cybermen enforcers. He may not have an entire army's worth of them, but only a few can still prove to be the deadliest new force in the universe. They may be gone for now, but the Master and his Cyber Masters will surely return, sooner or later.

Doctor Who stars Jodie Whittaker as the 13th iteration of the titular Time Lord. Whittaker is joined by co-stars Bradley Walsh, Tosin Cole and Mandip Gill in a series spearheaded by showrunner Chris Chibnall. The next episode, "Revolution of the Daleks," will air this holiday season. The new season and all previous 11 seasons will stream on HBO Max later this year.

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