If the chosen target becomes an illegal target for Flames of the Raze-Boar, the spell doesn’t resolve. No creatures are dealt damage.

If the target is dealt lethal damage this way, it will still be on the battlefield while Flames of the Raze-Boar deals damage to other creatures, if appropriate. Creatures dealt lethal damage won’t die until after Flames of the Raze-Boar is entirely finished resolving.