A portion of the game that forces you into open combat even if you were allowed to avoid it before.
Some games, particularly RPGs, allow the players to complete their objectives in a multitude of ways, be it straight-up combat, stealth, diplomacy, or something more exotic. Every so often, however (usually during Boss Fights or towards the end of the game), a game decides to suspend the players' right to avoid combat and dumps them in the middle of an open battle which they theoretically could handle--had they been developing the combat skills, rather than any other preferred path. If they had not, then alas, the battle catches them with their metaphorical pants down. In a way, the polar opposite of Stealth-Based Mission. Compare also Duel Boss and No-Gear Level, which can be similarly frustrating if you have been mainly relying on your party or powerful equipment in combat. Contrast RPGs Equal Combat, where every encounter is resolved by fighting without any alternatives.
Will go under Video Game Tropes.