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

The Mithril Long Sword is a long sword that can be wielded by players with at least level 20 attack. It can be made by players with at least level 56 smithing using 2 mithril bars, granting 100 smithing experience. It can also be bought at Varrock Swords.

Dropped by

They are dropped by:

Monster Combat level Quantity Rarity
Chaos Dwarf 59 1 4Rare