Dominia

A red card is a card with a red color bar (usually at the top of the card).

“When this attacks” is a triggered effect that triggers when the dragon is considered to have attacked, during the attack step. It does not trigger if the combat chain closes in the layer step of combat. After it triggers, it will still resolve even if the dragon ceases to exist or the combat chain closes.

If the revealed card is not a red card, or if you have no cards in your deck to reveal, the rest of the triggered effect fails to resolve.

Banishing a card is not an optional effect. You must banish a card if you are able.