repente

Repente is an adverb, "suddenly, without warning", and it stands on repens, repentis, "sudden". The adjective you will meet more often than the bare repens is repentinus, and there is a second adverb repentino doing the same job, so the family is larger than it needs to be.

Latin is not short of ways to say this. Subito, statim, protinus, extemplo and repente all crowd the same space, and prose authors alternate them for variety rather than for any fine shade of meaning; do not hunt for a distinction that is not there. One trap for English speakers: it has nothing to do with repent, which comes from paeniteo. Another: keep it apart from the forms of repo, "to creep", which is a different root entirely.