For the Night Hag's Nightmare Haunting ability, the rules state that the character gains no benefit from their long rest. Does this mean that abilities that can be used once per long rest are not refreshed?
As an example, if a Cleric has used all their spell slots before their Nightmare Haunting, will they get their spell slots back? Also, will they be able to change their list of prepared spells?
While on the Ethereal Plane, the hag magically touches a sleeping humanoid on the Material Plane. {snip} If these visions last for at least 1 hour, the target gains no benefit from its rest, and its hit point maximum is reduced by 5 (1d10). If this effect reduces the target’s hit point maximum to 0, the target dies, and if the target was evil, its soul is trapped in the hag’s soul bag. (MM p. 178)
No benefit from the long rest. What are the benefits of a long rest? From the rules on resting:
At the end of a long rest, a character regains all lost hit points. The character also regains spent Hit Dice, up to a number of dice equal to half of the character’s total number of them (minimum of one die). (PBH, p. 186; Basic Rules, p. 67)
They need rest—time to sleep and eat, tend their wounds, refresh their minds and spirits for spells for casting, and brace themselves for further adventure. (PBH, p. 186; BR, p. 67)
From the cleric's class description, under Preparing and Casting Spells:
You regain all expended spell slots when you finish a long rest. (PHB p. 58; BR, p. 22)
Regaining all expended spell slots looks like a benefit of a long rest. The prayer and preparation to change what is prepared are contingent on there being spell slots that need to be filled with spells: if the cleric has no spell slots regained, it is irrelevant to spend time in prayer changing the spells that are in those (non existent) slots.
I'd suggest that you discuss this with your DM to get their ruling. One could read the resting rule benefits as being confined to the Hit Dice/Hit Points recovery. If that is the ruling, then the preparation / change can go as spelled out in the same paragraph of the rules:
You can change your list of prepared spells when you finish a long rest. Preparing a new list of cleric spells requires time spent in prayer and meditation: at least 1 minute per spell level for each spell on your list. (Basic Rules, p. 22)
By this text, the ability to change what spells you have prepared for your spell slots is a benefit of the long rest.
As I interpret the Nightmare Haunting action from the Night Hag (MM pg. 178), yes, an affected PC's rest-rechargeable abilities and spell slots are not restored after taking a long rest.
However, I would say that changing prepared spells isn't really something you regain, a "benefit" of a rest, but rather something you can do, so I'd say you can still change prepared spells when affected by Nightmare Haunting.
The flavour of this feature seems to be around not letting you sleep properly, so you can't restore your health or abilities, but swapping prepared spells is a thing you do whilst awake, hence why I don't think it would be affected by Nightmare Haunting.
