Valley Flamecaller

If damage dealt by a source you control is being divided or assigned among multiple permanents an opponent controls or among an opponent and one or more permanents they control simultaneously, divide the original amount before adding 1.

Valley Flamecaller’s ability doesn’t cause it to deal damage; it affects the amount of damage dealt by the original source.

If another effect modifies how much damage your Lizard, Mouse, Otter, or Raccoon would deal, including preventing some of it, the player being dealt damage or the controller of the permanent being dealt damage chooses an order in which to apply those effects. If all of the damage is prevented, Valley Flamecaller’s effect no longer applies.