A lot of suggestions have been posted about a boss designed around Devilites, and I am a great supporter of this, and in response I have come up with several ideas that would be understandable rewards:

This first one was not originally mine, but I found it to be a great idea worth repeating, and if it was yours please step forward and I will happily give you credit.

Pay-day shield. A shield shaped like a briefcase, with a dollar sign or perhaps the fiend insignia on the face, with a hand full of cash hanging out of the edges, as well as a trail of the green stuff as you walk.
Stats would consist of a shadow defense (obviously) and maybe piercing (briefcases are think y'know? specially with all that mulah, and we need a shadow/piercing). Along with this i would suggest one of two of the following bonuses once the shield reaches a 5***** status. A 30% chance to double the amount of crowns gained from a single coin pick up (to the specific knight only) OR a unique charge that occurs when the shield has been long enough, money burst from the shield, dealing no damage but instead a knock-back.

Tie-Fighter (sword). A three swing series sword that should be broken down into three damage types, shadow piercing and normal, designed like a thee color striped tie. The shadow piercing and normal would each be a little greater than a third of a bar, and this imho would work our greatly with a lot of recent balancing issues that the setenza seems to have. Obvioulsy the blade would prove near null against slime, fiends and undead, but overall the normal damage would make up for this weakness, but not over-power the blade. gremlins and wolvers would recieve greater damage of course, but the idea is that you really really want to get a UV against these specific monsters, then this weapon would really shine. Perhaps make a few upgrades that go through seperate routes, like a 5***** version called "You're fired!* that has a chance to inflict fire status? As for the charge effect, basic charge up, and upon release, the knight swings out the sword and points it forward, in which it will grow in size and extend outward, not changing the size of the stripes, creating a fun optical illusion effect.

Last but not least, we must discuss the phearful and misery business that is coffee. A nice quaint coffee cup shaped bomb seems in order. I suggest a weak shadow damage, with a hint of both small chances of shock and fire status. Why shock and fire? Shock is representative of energy(caffeine), and fire, well you play with hot coffee, you get burned. IN ADDITION I purposely include these two so that this bomb will have a weakness to both quicksilvers and oilers. Why you ask? No other bomb has two status effects, and many might consider this overpowered, even with low shadow damage, so why not, make it weak against those pesky slimes that you have to really avoid helping? I think this would prove to be a great balance mechanic for such a fun weapon. In addition the 5***** variation of this must be called the 'Coffee mug of DOOM' with a large rotating "DOOM" on an electronic screen :3.