I was going to suggest that you look at In Nomine by Steve Jackson games for inspiration, but I guess that isn't necessary now.I want to thank you all for your suggestions. I had them hooked until the last possible moment and then they changed their mind and chose to take the combat with Vampiric Black Dragon they were striking a deal with regarding the Draconic Prophecy and instead find the information elsewhere, if possible.
I rarely misjudge my players after DMing with them for almost 20 years, and I really thought they were going to take the bait.
Maybe some other time.
Fetucide suggestion
While it's no longer relevant -- technically speaking, a rakshasa probably counts as a fallen angel.
And the PCs are obtuse enough to step right in it!Maybe the fallen angle is a go with the evil project.
Maybe it was all the angle's idea in the first place.
Maybe it's wheels within wheels and it's actually a way for the angles to find those who could fell one of their own and neutralize them.