The phrase "main antagonist" refers to the most important character or force that opposes the protagonist in a story or movie. They are usually the one causing conflict and obstacles for the main character to overcome. Full definition
Despite being the true main antagonist of the «Dark Seeker Saga,» Master Xehanort did not appear until Kingdom Hearts Birth by Sleep, the series's sixth installment, and he did not meet Sora face to face until Kingdom Hearts 3D: Dream Drop Distance, the seventh. (khwiki.com)
The second and last appearance of Hammer Bros. in the show was in the episode «The Trojan Koopa», where they appeared as main antagonists and act as guards of King Koopa's latest fortress. (mariowiki.com)
SEGA have shown off some more gameplay of Sonic Forces ahead of the game's release date of November 7, and while it showcases some interesting moments such as the beloved Chemical Plant Zone and a boss battle with main antagonist Infinite, what caught our eye is the introduction of a new «tilted» isometric camera perspective that the Sonic series has rarely used. (gamewatcher.com)