The "Force field" spell effect reiteration.

[Punainen]

To be honest, I always figured these spells were just an overly-visual Protect spell

I'm pretty sure it is protect also. Even in game, if you have protect on, you'll see a little flash of a shield appear when you're hit, showing it deflecting some of the damage. So I'm quite sure this is merely 'protect', and perhaps they made it a fog instead of a dome in the End of an Era trailer in order for it not to be confused with Louisoix's shield spell.