Combust can be targeted by spells and abilities that try to counter it (such as Cancel). Those spells and abilities will resolve, but the part of their effect that would counter Combust won’t do anything. Any other effects those spells and abilities have will work as normal.

If the targeted creature is an illegal target by the time Combust would resolve, Combust won’t resolve for having an illegal target.

Spells that create prevention effects affecting the targeted creature can still be cast, and abilities that create prevention effects affecting the targeted creature can still be activated. However, damage prevention shields (including those created before Combust was cast) don’t have any effect on the damage dealt by Combust. If such a prevention effect has an additional effect, the additional effect will still work (if possible).

If a static ability would prevent damage from being dealt to the targeted creature, it fails to prevent the damage dealt by Combust. If that ability has an additional effect that doesn’t depend on the amount of damage prevented, that additional effect will still work. It’s applied just once as Combust resolves.

Effects that replace or redirect damage without using the word “prevent” aren’t affected by Combust; they’ll work as normal.

If a creature is dealt lethal damage by Combust, it can still regenerate. If it does, the damage marked on it will be removed from it.

Combust : règles, fonctionnement et interactions (MTG)

Combust is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This spell can't be countered. Combust deals 5 damage to target white or blue creature. The damage can't be prevented.

How it works : Combust

Original English text — Wizards of the Coast / Scryfall

#1August 15, 2010· Wizards of the Coast
Combust can be targeted by spells and abilities that try to counter it (such as Cancel). Those spells and abilities will resolve, but the part of their effect that would counter Combust won’t do anything. Any other effects those spells and abilities have will work as normal.
No other card is named in this ruling.
#2August 15, 2010· Wizards of the Coast
If the targeted creature is an illegal target by the time Combust would resolve, Combust won’t resolve for having an illegal target.
No other card is named in this ruling.
#3August 15, 2010· Wizards of the Coast
Spells that create prevention effects affecting the targeted creature can still be cast, and abilities that create prevention effects affecting the targeted creature can still be activated. However, damage prevention shields (including those created before Combust was cast) don’t have any effect on the damage dealt by Combust. If such a prevention effect has an additional effect, the additional effect will still work (if possible).
No other card is named in this ruling.
#4August 15, 2010· Wizards of the Coast
If a static ability would prevent damage from being dealt to the targeted creature, it fails to prevent the damage dealt by Combust. If that ability has an additional effect that doesn’t depend on the amount of damage prevented, that additional effect will still work. It’s applied just once as Combust resolves.
No other card is named in this ruling.
#5August 15, 2010· Wizards of the Coast
Effects that replace or redirect damage without using the word “prevent” aren’t affected by Combust; they’ll work as normal.
No other card is named in this ruling.
#6August 15, 2010· Wizards of the Coast
If a creature is dealt lethal damage by Combust, it can still regenerate. If it does, the damage marked on it will be removed from it.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Combust.

Combustion Man

Whenever Combustion Man attacks, destroy target permanent unless its controller has Combustion Man deal damage to them equal to his power.

Combustion Technique

Combustion Technique deals damage equal to 2 plus the number of Lesson cards in your graveyard to target creature. If that creature would die this turn, exile it instead.

Liberating Combustion

Liberating Combustion deals 6 damage to target creature. You may search your library and/or graveyard for a card named Chandra, Pyrogenius, reveal it, and put it into your hand. If you search your library this way, shuffle.