I have a player in my new campaign who is going to play a bar/warlock, his general idea is to play it like jables from tenacious d, and he draws the power of his patron from a tooth of a pit fiend (the pick of destiny) and I thought it might be neat to give him a unique ability to fit with his multi class, I call it

"Demonic inspiration" What is was envisioning it as, was a slightly tweaked version of regular basic inspiration, but it has to have a positive and negative quality to it. Kind of like a deal with the devil type feel. My initial thought of it was as follows; starting at level 5 he would gain the ability of demonic inspiration, he can use it once per long rest, the demonic inspiration die would always be 1 die higher than a bardix inspiration die, so if his verdict inspiration die was 1d6, then demonic inspiration would be 1d10, when a player chooses to use the demonic inspiration, they must reroll their skill/attack roll and they must use that new roll, and add their demonic inspiration to that new roll. Anytime that his bardic inspiration die levels up, he would be able to use his demonic inspiration ability one more time per long rest, and the demonic inspiration caps out at a d12 just like bardic. Granted this is just my initial thought of the concept, I'm open to some advice