In order to do a ritual to activate an item of legacy, my party will need to slay a demon. Now, I want this to be a little unusual and exciting - it can be any demon, but I want the encounter to be special and memorable. I want it do be 'that time we activated Thaas by doing something really cool' and not 'that time we slew yet another demon and paid 1500 gp to get nifty powers on that bow'.
Any ideas for me? The encounter should include a demon between CR 11 and CR 14. It can include other creatures, but there's no need.
A little more detail on the item, the setting you're in, and the characters involved would help a great deal in motivating nifty ideas