Chastise 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

Destroy target attacking creature. You gain life equal to its power.

Complete guide

Chastise is a classic white reactive spell that punishes aggressive plays while bolstering your own life total. It serves as a powerful defensive tool that can swing the momentum of a game by removing a threat and undoing the damage dealt by previous attacks. This instant-speed removal spell ensures that an opponent's strongest attacker becomes your greatest source of healing.

What it Does

Chastise is an instant that targets a single creature currently declared as an attacker. Upon resolution, it destroys that creature and grants you life points exactly equal to the power that creature had as it left the battlefield. Because it specifies an 'attacking creature,' you can only cast this spell during the combat phase after attackers have been declared and before the combat phase ends.

Step-by-Step Resolution

First, you cast Chastise by paying its mana cost and selecting a legal target—a creature currently attacking you, a teammate, or a planeswalker you control. Second, as the spell resolves, it checks if the target is still legal; if the creature was removed or stopped attacking, the spell fails. Third, the creature is destroyed and put into its owner's graveyard. Finally, you check the creature's power at the moment of its destruction and gain that much life.

Common Mistakes to Avoid

A frequent error is trying to use Chastise on a creature before it is declared as an attacker, such as during the Precombat Main Phase. You also cannot target a creature that has 'vigilance' unless it has actually been declared as an attacker, even if it hasn't tapped. Additionally, if the creature has 'Indestructible,' Chastise will fail to destroy it, but you will still gain life equal to its power because the life gain is not contingent on the creature successfully dying.

When to Play It

The best time to cast Chastise is usually during the 'Declare Blockers' step or the 'Combat Damage' step. By waiting until after you have declared your own blockers, you can see if the opponent uses any combat tricks to pump their creature's power. Since you gain life equal to the creature's power, allowing your opponent to boost their creature first actually results in you gaining more life.

Interaction examples

Turning the Tide

Generous Gift

If an opponent attacks you with a massive creature, you can cast Chastise to destroy it and gain life. If you later need to handle a non-attacking threat, you can use Generous Gift to destroy any permanent. Note that if you use Generous Gift to destroy your own land to create a 3/3 Elephant, that Elephant could eventually be a target for your own Chastise if an opponent gains control of it and attacks you!

The Protection Problem

Teferi's Protection

If you target an attacking creature with Chastise and the opponent responds with Teferi's Protection, all of their permanents phase out. Since phased-out permanents are treated as though they don't exist, Chastise will lose its target and fail to resolve. You will not destroy the creature and you will not gain any life.

Life Gain Timing

Imagine a 5/5 creature is attacking you. If you cast Chastise during the Declare Attackers step, the creature is destroyed before it deals any damage. You gain 5 life and your life total is effectively higher than when the turn started. If you wait until after damage is dealt, you will take 5 damage first, then gain 5 life back when you cast the spell before the end of the combat phase.

Guide written with AI from the official text and rulings. Official English text prevails.