Bonfire of the Damned’s mana value is based on its mana cost of , even if you’re casting it for its miracle cost. For example, if you cast Bonfire of the Damned for its miracle cost and choose 4 for X, its mana value is 9.

Preventing some or all of the damage dealt to the player won’t affect the amount of damage dealt to each creature that player controls. The same is true for any effect that increases or decreases the amount of damage dealt to the player.

A mana cost of means that you pay twice X. If you want X to be 3, you pay to cast Bonfire of the Damned.

If the player is an illegal target when Bonfire of the Damned tries to resolve, the spell won’t resolve and none of its effects will happen. No damage will be dealt to any permanent or player.

Bonfire of the Damned targets only the player. Creatures with shroud or hexproof that player controls will be dealt damage by Bonfire of the Damned if it resolves.