The phrase "climactic showdown" refers to the final and most intense confrontation between opposing forces. It often involves a crucial or decisive moment in a story or conflict. Full definition
As you might imagine, Legacy of the Void acts as a culmination of all the events and teases which were shown in both Wings of Liberty and the 2013 second act Heart of the Swarm, leading into a final climactic showdown with a big baddie who hates the Terran, Zerg and Protoss in equal measure. (gamecrate.com)
At least three of its images are clichéd enough to have already been parodied on The Simpsons: the hero silhouetted against the sunset as he carries his girl to safety; the aforementioned climactic showdown on top of a nuclear reactor (unfortunately, unlike Homer, no character here is wearing a muumuu); and the hero shouting «Nooooo!» (slate.com)
But it's all leading to the final, climactic showdown at Hogwarts. (bigpicturebigsound.com)