From Pixelmon Wiki
Aurora Veil is an Ice-type move that halves the damage done to the user by both physical and special moves for five turns. In double battles, Aurora Veil reduces the damage of moves for the user and their ally by a third rather than a half.
Aurora Veil can only be used if Hail is active. The duration of Aurora Veil is extended to 8 turns rather than 5 if a Light Clay is being held. An Icium Z will raise the user's speed stat by one stage and make the move Z-Aurora Veil.
If a Pokémon has the ability Infiltrator, the effects of Aurora Veil will be ignored when attacking.