致命(zhì mìng) - fatal, lethal, deadly in Mandarin

致命 (zhì mìng) describes something that kills: a wound, a poison, a disease, a blow. Grammatically it is a verb-object compound doing adjective work, so it almost always appears with 的 in front of a noun, or after 是 at the end of a clause.

Like English fatal, it stretches happily into the figurative. 致命的错误 is a fatal mistake in a plan, 致命的打击 a devastating blow to a business. 致命弱点 is the fatal flaw, the Achilles heel.

The trap is trying to use it as a plain predicate the way you would 危险. 很致命 is possible but sounds odd; Mandarin prefers 是致命的. And keep it separate from 要命, which looks similar but is an exasperated colloquialism meaning something is a nightmare, not that anybody actually dies.