Do you have to pay power costs of a unit played from Rift Herald's Deathknell effect?

Yes, you must pay the Power cost when playing a unit via the Rift Herald Deathknell effect.

Ruling

According to the text of Rift Herald, the Deathknell effect allows you to play a unit from your hand to your base while ignoring its Energy cost, but it explicitly requires you to pay the unit's Power cost.

How it Works

1. Trigger: When Rift Herald dies, its Deathknell ability triggers and is placed on the chain (Rule 808.1.d.2).
2. Resolution: When the Deathknell ability resolves, you choose a unit in your hand to play to your base.
3. Cost Determination:
   • The Deathknell effect instructs you to ignore the unit's Energy cost. Per Rule 356.1.b.2, when an instruction tells you to ignore one component of a card's cost (Energy), only that specific component is set to zero.
   • The remaining cost—the Power cost—still applies as dictated by the card's text.
4. Payment: During the process of playing the card (Rule 357), you must pay the required Power cost to successfully put the unit into play.

If you are unable to pay the Power cost, you cannot complete the action of playing the unit.

Note: I could not fully verify this ruling against the retrieved rules.