Guardian of the Shadowrealm (Red)

Action -

: Return this to your hand. Activate this only while this is in your banished zone.

Blood Debt

Rulings

From the Monarch release notes

Since defense reactions can’t defend in the defending window and must be played in the reaction window, you must pay Guardian of the Shadowrealm’s cost (2) to play it.
The action ability can only be used when Guardian of the Shadowrealm is in your banished zone.
The action does not have go again and requires an action point.
Guardian of the Shadowrealm goes into the graveyard when the combat chain closes after it is played.

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.