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

A Mithril Throwing Dart is a throwing dart which can be obtained by smashing Strange Barrels or by attaching feathers to mithril dart tips with level 52 fletching. Granting 1 fletching exp per dart created. It requires level 15 Ranged to equip.

Poisoned Mithril Throwing Darts can be created by using weapon poison on them.

Like all throwing darts, they provide no WeaponAim or WeaponPower, and when they are equipped, they are invisible to other players. However, they provide a significant invisible ranged bonus.