The Nightmare Chest is a limited time treasure chest offered in Castle Age. It costs 30 FP for one item pull from the chest and 80 FP for 3 pulls. It is available for players of all levels.
It was introduced on September 28, 2018 with a limited time of 168 hours (1 week) to use it.
It also differs from the other chests in that there is a "Legendary" section. You have a 1% chance to pull a "Legendary" reward.
Note: It appears you get more than one item when you get a drop from this tier, even though it isn't indicated in game (in fact the text after rolling even says otherwise):
Evolution Crystals: x3
Hero Potions: x3
Refine Crystals: x2
Enhance Crystals: x2
Occurrences[]
Sept. 28, 2018 with a limited time of 168 hours (7 days)
Dec. 26, 2018 with a limited time of 144 hours (6 days)
Feb. 24, 2019 with a limited time of 120 hours (5 days)
Apr. 4, 2019 with a limited time of 120 hours (5 days)
Dec. 28, 2019 with a limited time of 144 hours (6 days)
You get more than one item when you get a Common tier drop, at least since the 5th occurrence of this chest. It might have been the case before, but it was never reported (the text when you get a drop says you get only one item even though you actually get more). It might also have been the case with the Anniversary Chest.
Introduced on September 28, 2018 at 0:00 PST (7:00 am UTC on the 28th).
To check when this event occurred in the past, see this page: Events Rotation.