I would suggest reactivity in the world. Something like:
1. After getting to the grove, if you long rest five or more time then the ritual is complete. The tieflings are cast out and the grove is forever closed. Maybe you find dead tiefling bodies every so often. "Goblin gotcha?"
2. If you go somewhere like the goblin camp and attack then retreat to go long rest... when you return the goblin camp is reactive. There are more guards, they're more vigilant, maybe the leaders have gotten together in conference which would make them harder to confront. People coming in are questioned more fully.
That sort of thing. Make the world progress in measurable ways as the long rests continue.
+1 for world reactivity.
For your grove example, there should definitely be some sign that the ritual is progressing. Louder chanting, magical effects appearing, dialogue changes. That'd be super cool.
