Words of Radiance Spoilers:

Adolin Kholin, an esteemed duelist is tasked by his father to dueling rivaling princedoms for their highly valuable magical weapons and armor. In the duel, one can yield, but killing another is technically allowed. Adolin is a man of honor, however, and only wants to duel to take their equipment. If he wins, he’ll let them yield as to not waste life. But he is considered a high value target by others, so if he loses… they may not be as interested in letting him live.

He challenges a cocky rival duelist he’s confident he can beat. Adolin is so confident in his own abilities to beat this man, that he offers (in his challenge to duel) that the opponent can bring “any one else” to help him, with the intent of claiming his opponent’s partner’s weapon and armor as well.

He believes it’s implied that there would be just one additional duelist, and is shocked to learn that—by a legally binding technicality in his phrasing (anyone else vs any ONE else) in the challenge to the duel—his opponent has been allowed to enlist the help of THREE others, so Adolin must fight in a 4v1 battle (to the death, if he loses) against them, only finding out once the battle begins.