Curse. Once you don this cursed armor, you can't doff it unless you are targeted by the remove curse spell or similar magic. While wearing the armor, you have disadvantage on attack rolls against demons and on saving throws against their spells and special abilities.
Xanathar Guide to Everything, Chapter 2: Sleep Sleeping in Armor: Sleeping in light armor has no adverse effect on the wearer, but sleeping in medium or heavy armor makes it difficult to recover fully during a long rest. When you finish a long rest during which you slept in medium or heavy armor, you regain only one quarter of your spent Hit Dice (minimum of one die). If you have any levels of exhaustion, the rest doesn’t reduce your exhaustion level.
Demon Armor: has a curse attached to it.
Curse. Once you don this cursed armor, you can't doff it unless you are targeted by the remove curse spell or similar magic. While wearing the armor, you have disadvantage on attack rolls against demons and on saving throws against their spells and special abilities.
Xanathar Guide to Everything, Chapter 2: Sleep
Sleeping in Armor: Sleeping in light armor has no adverse effect on the wearer, but sleeping in medium or heavy armor makes it difficult to recover fully during a long rest. When you finish a long rest during which you slept in medium or heavy armor, you regain only one quarter of your spent Hit Dice (minimum of one die). If you have any levels of exhaustion, the rest doesn’t reduce your exhaustion level.
Was the curse intended to be this lethal ?
No. And it isn't by itself, even with the optional rule from xanathars.
thanks for the info