Backdraft Hellkite

If a card with no mana cost gains flashback, it has no flashback cost. It can't be cast this way.

Wotc — March 19, 2021

If a card has multiple instances of flashback, you may choose any of its flashback costs to pay.

Wotc — March 19, 2021

If a split card gains flashback, you pay only the cost of the half you're casting.

Wotc — March 19, 2021

"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."

Wotc — March 19, 2021

You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.

Wotc — March 19, 2021

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

Wotc — March 19, 2021

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.

Wotc — March 19, 2021

You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.

Wotc — March 19, 2021

If a card with flashback is put into your graveyard during your turn, you can cast it if it's legal to do so before any other player can take any actions.

Wotc — March 19, 2021

If you cast an instant or sorcery with {X} in its mana cost this way, you still choose the value of X as part of casting the spell and pay that cost.

Wotc — March 19, 2021

If you cast a spell with flashback, you can't pay any alternative costs such as overload costs. You can pay additional costs such as kicker costs. If the spell has any mandatory additional costs, you must pay those to cast the spell with flashback.

Wotc — March 19, 2021

Once Backdraft Hellkite has attacked, its ability will resolve and grant the appropriate cards flashback even if Backdraft Hellkite leaves the battlefield before the ability resolves or after it resolves but before you cast the spells.

Wotc — August 23, 2019

Only instant and sorcery cards in your graveyard when Backdraft Hellkite's triggered ability resolves will gain flashback. Instant and sorcery cards that are put into your graveyard later in the turn won't gain flashback.

Wotc — August 23, 2019