Shaden Death Hydra (Yellow)

When this attacks, it deals X damage to you, where X is 13 minus the number of cards with blood debt in your banished zone.

Blood Debt

Rulings

From the Dusk till Dawn release notes

If your hero’s life total is reduced to 0 when you attack with this, you lose the game before this deals damage to the attack target.
If you have more than 13 cards with blood debt in your banished zone, you are not dealt any damage. You can not be dealt negative damage.

Keyword notes

Blood Debt - Blood Debt is a new keyword existing on Shadow cards. Shadow cards can inflict a loss of life to access a greater power. At the end of your turn, you lose 1{h} for each blood debt card in your banished zone.

You only lose life at the end phase. Players do not have priority during the end phase.
If you have multiple cards with blood debt, you lose 1 for each instance of blood debt at the end of turn.
Only face-up cards banished with blood debt counts and will lose you . Face-down banished cards have no effects or stats.