The sinister chest is a locked chest located in the Agility Dungeon in Yanille. It is in a part of the dungeon that requires 49 Agility to access. The chest can only be unlocked with the sinister key, which is a drop from Salarin the twisted and the Chaos Fanatic or a possible loot from a magpie impling jar/magpie impling. If opened, the chest will emit a toxic gas, which poisons the player so it is advised to bring an antipoison or other method of curing or preventing poison.
- Repeatedly using keys on the chest stalls the poison effect, thus retaining your health until you leave (assuming the skeletons are not aggressive)