OSRS dark key is contained in a chest in Isle of Souls dungeon. Once you claim it, you can use the key to open dark chest for 1,500 Thieving XP and an item from a new loot table. 

How to obtain OSRS dark key? 

To get hold of dark key, you have to get to Isle of Souls dungeon to open a chest. Isle of Souls dungeon is located on the northeastern exterior of the Soul Wars Arena. Enter the dungeon and come to the hallway. You will find a locked chest between the two dragon rooms. 
To open the chest, you have to reach level 28 Thieving. If you fail to unlock the chest, you will be sent out of the dungeon. Therefore, you'd better use a lockpick to increase the chance of opening the chest. Once opened, you will receive a dark key, 150 Thieving XP and other items.  

How to use OSRS dark key? 

When you claim OSRS dark key, you can be able to use it to open dark chest. Dark chest can be found in the basement of Crumbling Tower on the northwest coast of Isle of Souls. After opening the chest, you will get 1,500 Thieving XP and an item from followings:
Rune pickaxe
Rune axe
Dragonstone
Dark key
Rune longsword
Chaos/Blood/Nature runes
Green/Blue/Red/Black dragon leathers
Grimy dwarf weed
Grimy torstol
Grimy ranarr weed
Grimy snapdragon 
Babydragon bones
Dragon bones
Adamant arrow
Rune arrow
Coins 

Hope now you know how to obtain OSRS dark key to open dark chest. Furthermore, you can always buy OSRS gold and others from us. 

The RS3gold Team