RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Adamantite dagger
The Old School RuneScape Wiki also has an article on Adamantite dagger.

An Adamantite dagger is the second most powerful dagger in RuneScape Classic, behind the Rune dagger. They can be made at level 70 Smithing from 1 adamantite bar, yielding 62.5 experience, or purchased from Varrock Swords.

This weapon has a poisoned variant, which can be created by using weapon poison on it.