Volcanic Salvo
Sorcery
This spell costs 
less to cast, where X is the total power of creatures you control.
Volcanic Salvo deals 6 damage to each of up to two target creatures and/or planeswalkers.
Seeing it coming a mile away is little comfort if you can't get out of range in time.
Illustrated by
Core Set 2021 Promos#172pRareEnglishLegality
Rulings
To determine the total cost of a spell, start with the mana cost or alternative cost you’re paying, add any cost increases, then apply any cost reductions (such as that of Volcanic Salvo). The converted mana cost of the spell remains unchanged, no matter what the total cost to cast it was.
The cost reduction applies only to generic mana in the cost of Volcanic Salvo. It can’t reduce the requirement.
Once you begin to cast Volcanic Salvo, players can’t take other actions until you’ve finished casting it. Notably, they can’t try to change the total power of your creatures.
The total cost to cast Volcanic Salvo is locked in before you pay that cost. For example, if you control three 2/2 creatures, including one you can sacrifice to add 
to your mana pool, the total cost is 
. Then you can sacrifice the creature when you activate mana abilities just before paying the cost.
If a creature’s power is somehow less than 0, it subtracts from the total power of your other creatures. If the total power of your creatures is 0 or less, the cost remains .
You can’t target the same creature or planeswalker twice with Volcanic Salvo to deal 12 damage to it.