What is the "Zenithar's Blessing" Buff in ESO?

In the current Zeal of Zenithar event, some players will have noticed a buff in the Active Effects section of their Character menu, called Zenithar's Blessing. To begin with, nobody was too sure what this buff was for, but this has now been discovered and will be explained in this article. Beware that there are some minor spoilers here related to the special world encounters that are active during the event, so if you'd rather be surprised by these events in-game then it's probably best that you don't read on. 

The Zenithar's Blessing buff in ESO

What is Zenithar's Blessing in ESO?