Eltab
Lord of the Hidden Layer
OP:
The 4e Draconomicon had a 'Dream Dragon' (or 'Nightmare Dragon', I forget) whose fluff might serve your purpose.
This high-end nasty tries to put you to sleep rather than kill you outright.
You could have something similar that haunts the PCs' dreams, turning thoughts into nightmares - and messing up their Long Rest. The longer the PCs stay on the Plane of Time - and don't defeat the dragon - the less effective rest / recovery becomes. The PCs are now effectively on a time limit, eventually exhaustion will be a permanent condition for them.
The 4e Draconomicon had a 'Dream Dragon' (or 'Nightmare Dragon', I forget) whose fluff might serve your purpose.
This high-end nasty tries to put you to sleep rather than kill you outright.
You could have something similar that haunts the PCs' dreams, turning thoughts into nightmares - and messing up their Long Rest. The longer the PCs stay on the Plane of Time - and don't defeat the dragon - the less effective rest / recovery becomes. The PCs are now effectively on a time limit, eventually exhaustion will be a permanent condition for them.