The mysterious bottle is an item that can be unlocked from 3,000 Sliskean reputation in the Heart of Gielinor.
When opened, it will trigger a small minigame where a small bank of black mist will escape from the bottle, and the player will have to walk close to the mist and use the bottle to try to capture it, although the player will fail 10 times before the player can capture the mist in the bottle. The game will end if the player fails to capture the mist within about 22 seconds. Once completed, it will tell the player their time and whether they have set a new best time or not. The bottle can tell the player their best time.
Trying to activate the bottle in an enclosed area that is too small to play the game will give players the text "You open the bottle but the strange mist within lies dormant."
Together with the other Heart of Gielinor toys, it is part of the Mod Deg trading sequence by using one of them, while having the rest of them in the inventory, on the underwater disguise kit to make the broken disguise kit.