We started at level 4 and for the final major encounter of level 4 I put the 5 person party against a Smoke haunt. My team was fully healed, but were fatigued, had no spells, and are in an arctic climate, so they had to crowd around a fire or roll survival checks. They had to complete one last guard duty together as a group before turning in for the night.

I told the players that they were going to have to make a variety of rolls to stay awake, and to use whatever means they could to achieve that goal. They had to roll perception every round due to the fact that they were on guard duty, and I'd be having them roll a variety of checks to see how well they are able to stay awake. The perception rolls of course were to detect the smoke haunt, and the various will, and fort saves were for spells, or abilities the smoke haunt was using. So in the first round nothing really happened, the smoke haunt dealt some damage, and was undetected. In the second round one of the players passed their perception, so I rolled a d100 publically (for no real reason) then said that he saw a face in the fire staring back at him. I let him react in character, then used the rest of the party as an ally, and told them that when they look, they don't see a face, so that person may be starting hallucinate. That person spent the next 3 rounds trying to convince everyone else in the party of what they saw while another was restraining him. One of my characters "fell asleep", and another (A Dhampir that was getting healed by it) ended up seeing it, but just kept staring back because it seemed to be empowering him. The character that was asleep almost died, but received last minute care, and finally after about 6 rounds, the fifth character threw snow onto the fire culling the flames enough for everyone to see the creature clearly at which point it teleported away.

I knew this would be a fun encounter from the get-go, but since they thought it was just roleplay, the players RP made it into a great encounter. I didn't want to make this overly long, so if you want my notes on exactly how I ran this from a DM perspective, just let me know, and I'll post it in the comments.