A climactic, plot-important Boss Battle, usually occurring after a major revelation in the plot (e.g., about 2/3 into the game). Naturally, this means higher stakes and a more dramatic tone than your regular boss fight. The boss in question is also likely to be a major figure in the overall plot or a personal enemy of one of the main characters instead of some sort of one-scene monster. If you suddenly start hearing Battle Theme Music that's not the usual one used for boss battles and you know you're not even close to the Final Boss, then you're definitely dealing with one of these.
Note that to qualify for this trope, the boss must be at least somewhat of a challenge; if they're a complete pushover, that's Anti-Climax Boss. However, the boss fight does not have to be overly difficult, and a particularly difficult boss fight does not qualify as a Climax Boss unless it comes at a climactic point in the plot. Final Bosses also do not qualify, nor do bosses fought very close to the Final Boss (unless said boss was a constant presence throughout the story, meaning that you're finally getting some payoff for all that buildup); ideally a Climax Boss would take place somewhere around the two-thirds or three-quarters mark, not nine-tenths.
See Pre-Final Boss for bosses that come right before the final boss, Wake-Up Call Boss for the first hard boss fight early in the game and Hopeless Boss Fight when there is no way in hell the boss will be beaten just because of plot. Also differs from a Disc-One Final Boss in that the Climax Boss is not presented as the final boss despite being more dramatic than the usual boss.
Not to be confused with Pivotal Boss, which is a boss that the battle literally pivots around, like a wheel and its axle. Contrast Anti-Climax Boss, That One Boss, Final Boss. Inversion is The Unfought, where this kind of battle is set up, but never happens. It isn't usually required to beat them in Orgasmic Combat.
