Lesnar appeared when John Cena invited The Rock out to close the show. During Cena’s promo, the crowd had chanted for Lesnar twice.
Lesnar laid Cena out with the F5 to end the show.
UPDATE: Brock Lesnar showed up to RAW literally 30 seconds before he appeared on TV.
This probably explains why it seemed like John Cena was stalling during his promo.
Jim Ross tweeted the following regarding Brock Lesnar's return after RAW: "Why's Brock Lesnar back? I'd suggest 2 earn BIG money. He's done w/ UFC & still young. He's a complex, supreme athlete. Interesting times. Never full time IMO. But enough to matter."
UPDATE x 2: John Cena called The Rock out to the ring on Monday's RAW SuperShow, but what he got instead was something entirely different: Brock Lesnar.
Lesnar's theme music hit to a thunderous applause. Cena stared toward the entrance ramp as Lesnar slowly emerged through the backstage area and appeared on stage. He bounced around and flexed his chest muscles before pacing the stage while Cena smirked. Lesnar continued to walk around the stage before slowly making his way toward the ring. Lesnar paced around the ring, then slowly made his way into the ring as Cena applauded.
Lesnar came face-to-face with Cena and extended his hand, but suddenly hoisted Cena up on his shoulders. Lesnar turned around and hit Cena with the F5. Michael Cole declared that Brock is back and that the landscape of Monday Night Raw has changed.
Here is another video just in case the one above gets pulled:
....... and another: