formorti

Another clean decode, and the prefix does all the work.For is a full adverb meaning away, and Esperanto lets you weld it to the front of a verb to add removal or disappearance. Morti is to die. Die plus away gives formorti: to die out, to become extinct, to dwindle to nothing. It is not one death but the end of a whole line, so a person mortas and a species, a language, a custom or a family name formortas.

The for- family is worth collecting in one go.Forviŝi is to wipe away, forpreni to take away, fordoni to give away, forĵeti to throw away, forporti to carry away, foriri to go away. Learn the prefix once and half a dozen verbs arrive together. Forigi is the neutral general-purpose one, simply to remove.

Grammar note, because it matters here. Morti is strictly intransitive and formorti inherits that: things die out, nobody dies them out. To wipe a species out deliberately you need -ig-, giving formortigi, to exterminate. The participle formortinta describes something that has died out already, which is how you say an extinct species: formortinta specio. And the noun is formorto, the dying out itself.