SPOILER WARNING: This article contains major spoilers for "The Last Temptation of Barry Allen, Pt. 2," the midseason finale of The Flash Season 6.
In "The Last Temptation of Barry Allen, Pt. 2," the midseason finale of The Flash Season 6, Barry defeated Bloodwork in the battle for Central City. However, Dr. Ramsey Rosso didn't go out without a fight, and he revealed his final form in the process.
Over the course of the episode, Rosso revealed his grand plan. He wanted to use STAR Labs' particle accelerator to disperse his blood across Central City and turn all its citizens into his own personal zombie army, thereby giving them "eternal life." He used his control over Barry to try and achieve this end, but he didn't count on his connection working two ways. Barry was able to plant some subtle suggestions in Rosso's mind, which allowed him to reveal Bloodwork's grand plan to Team Flash.
Once Iris and Cisco realized what Rosso wanted, they were able to counteract it. They recruited Allegra Garcia, Iris' Central City Citizen intern, who is a metahuman with the ability to generate UV light. Even though Rosso was successful in launching the particle accelerator, she used her powers to nullify the effect of his blood. The subsequent blast cured the infected victims, instead of infecting even more people.
The explosion gave Rosso time to flee, but he wasn't fast enough to outrun the Flash. Barry confronted Rosso, who went berserk with rage. His powers flowed through him, transforming him into a monstrous red hulk with a skull for a face. His new look echoed the character's look from the comics, with a few tweaks like fangs and bulbous skin protrusions.
Even though Barry was cured, he still had a connection to Rosso. He used that to do what Rosso had done to him in "Part 1": he projected an image of Rosso's mother, who expressed her profound disappointment in how he reacted to her death. The moment caused Rosso to revert back to his human form. Once Rosso's guard was down, Barry rushed him over to the Pipeline and locked him in an anti-metahuman cell.
With that, the Flash saved Central City once again -- and perhaps for the last time. After all, he is due to sacrifice his life in "Crisis on Infinite Earths," next week's major Arrowverse crossover. Knowing this, he spoke to his friends and his family and told them goodbye. As they counted down the minutes, the sky turned red, a sure sign that "Crisis" has officially begun.
Airing Tuesdays at 8 p.m. on The CW, The Flash stars Grant Gustin, Candice Patton, Carlos Valdes, Danielle Panabaker, Tom Cavanagh, Jesse L. Martin, Danielle Nicolet and Hartley Sawyer.