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

Mithril daggers are the third most powerful daggers in RuneScape Classic, being more powerful than Black daggers, but less powerful than Adamantite daggers. The can be made from one mithril bar at level 50 smithing, yielding 50 smithing experience, or purchased from Varrock Swords.

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