An adverb used to emphasize that something will certainly happen; similar to 'definitely' or 'surely' (somewhat literary/formal).
彼は必ずや成功すると信じている。
He believes he will surely succeed.
必ずやこの約束を守ってください。
Please be sure to keep this promise.
指導者は必ずや解決策を見つけるだろうと語った。
The leader said he would surely find a solution.