For the "shot in the back" trope I wanted a flavour of a fairly hefty burn spell. I went back and forth over lots of iterations: basically anything that can deal four or five damage is pretty good in limited. But there's a big gap between Flame Slash and Chandra's Outrage.

And then I decided to make it modal even though I originally didn't intend to, because it lets it often do four damage, but has a flavour of "always good, but deadly when someone lets their guard down". And appropriately vigilant creatures don't let their guard down :)

I've changed my mind, I think "shot in the back" should be a black spell, and I included assassinate, so this should be something other than tapped-conditional (unless I make that a theme). Not sure what to replace it with, though.