Am I about to slaughter my players?

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.