My players may soon be coming upon one of the reoccurring villian's temporary base of operations. My idea for this final encounter was at the top of this abandoned forts central tower would be where the villian was staying and left a nasty surprise of 2 Imps and a Spined Devil for whoever wonders into his old home. The idea is for the spined devil to rain spines on the players from the rafters while the 2 Imps disappear and reappear with their invisibility. Creating a very hectic encounter in a small environment. The party is 4 lvl 2s. And fairly inexperienced with DnD
Am I about to slaughter my players.
I'd like to give them a really hard challenge and then give them 1 or 2 magic items.
I think the players will be fine. If your really concerned have lots of boxes or tables that they can get cover from, also give them a way to escape (laundry/trash chute etc)
Another thing you can do is bait a powerful attack. Have an iron wrought chandelier hanging over the devil (the chain is ac 10/ 10 hp) with the quick release somewhere a little difficult to get to.
Or the tower is old and worn, and a well placed attack on a support beam will do a similar of the masonry. Keep the damage to both around 3d10 or so (don't give a save to the demon since you want to reward the creative way to attack )