Warning 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

Prevent all combat damage that would be dealt by target attacking creature this turn.

How it works : Warning

Original English text — Wizards of the Coast / Scryfall

No additional ruling has been published for this card.

Interactions with other cards

These cards' text mentions Warning.

Stonehide Ancient // Warning Tremor

Flying, Vigilance When this creature enters, return all non-Dragon creatures to their owners' hands. Warning Tremor deals 2 damage to any target. The next Dragon spell you cast costs {2} less to cast.

You Line Up the Shot

Choose one — • Aim for the Wyvern — Conjure a card named Plummet into your hand. • Aim for the Cursed Amulet — Conjure a card named Naturalize into your hand. • Fire a Warning Shot — Draw a card.