Ben enlists Kevin’s help to break the Omnitrix’s physical seal. The experiment backfires spectacularly:

Because Diamondhead is composed of nearly indestructible organic crystals, he is the perfect counter to Vilgax’s brute force. Ben manages to defeat Vilgax, forcing the conqueror to retreat and sparing the Earth. Why This Episode Matters

By the end of the episode, the status quo is shattered. Ben has saved the world, but at the cost of his best friend’s humanity and the trust of his mentor, Azmuth.

Instead of unlocking Master Control, the Omnitrix resets, and several of Ben’s alien forms (including Chromastone, Goop, Way Big, and Spider-Monkey) are physically ejected from the watch as independent, mindless entities.

Knowing his current roster isn't strong enough to stop this version of Vilgax, Ben ignores Azmuth’s warnings and attempts to to unlock the Master Control. The Turning Point: The Hack Gone Wrong