The adverb of unprecedented: to a degree never reached before. It nearly always sits in front of an adjective, and almost always one of size or amount — unprecedentedly high prices, unprecedentedly low temperatures, unprecedentedly large crowds. That is because "never before" is easiest to prove about numbers, so this is the sense of the parent word that survives into the adverb. It can also modify a verb ("the market unprecedentedly closed early"), but that use is rarer and reads as heavy. This is a long, formal, written word — you meet it in research papers, economic reports and serious journalism, and hardly ever in speech, partly because six syllables is a mouthful. Even good writers often step around it and say "higher than ever before" instead, which is shorter and lands harder. If you are choosing, that plain version is usually the better choice; keep unprecedentedly for formal writing where the compact form is worth its weight.

空前地、史無前例地。多半接在形容詞前面，例如 unprecedentedly high 就是「高到前所未有」。中文其實不太用這種副詞句型，寫成「高到史上最高」「創下歷史新高」反而自然。