Mutual Destruction — ruling 1

Once you announce that you’re casting Mutual Destruction, players can’t try to remove your permanents with flash to make it lose flash until you’re done casting it. If it loses flash after it’s been cast, it will still resolve if able.

Mutual Destruction — ruling 2

You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.

Mutual Destruction — ruling 3

The last time that the game checks whether you have permission to cast a spell is before costs are paid. If you sacrifice a creature with flash to cast Mutual Destruction and then have no other permanents with flash, you’ll legally finish casting Mutual Destruction even though it no longer has flash. In this case, abilities that trigger whenever you cast a spell with flash won’t trigger.

Mutual Destruction : règles, fonctionnement et interactions (MTG)

Mutual Destruction 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 has flash as long as you control a permanent with flash. As an additional cost to cast this spell, sacrifice a creature. Destroy target creature.

How it works : Mutual Destruction

Original English text — Wizards of the Coast / Scryfall

#1April 17, 2020· Wizards of the Coast
Once you announce that you’re casting Mutual Destruction, players can’t try to remove your permanents with flash to make it lose flash until you’re done casting it. If it loses flash after it’s been cast, it will still resolve if able.
No other card is named in this ruling.
#2April 17, 2020· Wizards of the Coast
You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.
No other card is named in this ruling.
#3April 17, 2020· Wizards of the Coast
The last time that the game checks whether you have permission to cast a spell is before costs are paid. If you sacrifice a creature with flash to cast Mutual Destruction and then have no other permanents with flash, you’ll legally finish casting Mutual Destruction even though it no longer has flash. In this case, abilities that trigger whenever you cast a spell with flash won’t trigger.
No other card is named in this ruling.