Gandalf's beams of light

[Iarhen]

In the book The Return of the King, Gandalf uses some kind of piercing light that shoots up from his hands.

He uses such light against 3 objectives: the Nazgul, the Witch King and
the Mouth of Sauron. These 3 beings are afraid and, probably, hurt by
such light.

But what kind of light is this one? What effect does it have on the enemie's servants? Because the Nazgul flee from it... and when the Rohirrim arrived, something similar to a thunderbolt arose from Minas Tirith, when the Witch King was defeated by Gandalf. And the Mouth of
Sauron screamed and fled when Gandalf shot it to rescue Frodo's things from his hands.

Is it produced by Gandalf's maiarin nature? Or by Narya? A combination of them? Does it reveal their insides to themselves?

Was it holy? If it was, then maybe such holiness was the one thing that affected the nasties... the spells that knit "their unseen sinews" will..