Wildfire Howl

If the gift was promised and the target is illegal as Wildfire Howl tries to resolve, it won’t resolve and none of its effects will happen. No creatures will be dealt damage.

Some instant or sorcery spells require alternative or additional targets if the gift was promised. You ignore these targeting requirements if the gifts aren’t promised for those spells. On the other hand, you can promise a gift for a permanent spell even if you won’t be able to choose targets for an enters ability of that permanent once the spell resolves.

For permanent spells with gift, an ability triggers when that permanent enters if the gift was promised. When that ability resolves, the gift is given to the appropriate opponent.