If I sacrifice my Riftherald and don't have hand cards after sacrifice, what resolves first: sacrifice effect or Riftherald play effect?

Resolution Order: Rift Herald and Sacrifice

When playing Sacrifice, its additional cost requires you to kill a friendly [Mighty] unit. If you choose Rift Herald, its Deathknell ability triggers. Because triggered abilities are placed on the chain immediately after the cost that triggered them is paid, the Deathknell effect is placed on the chain after the Sacrifice spell has been announced but before the play process is finalized. Consequently, the Deathknell effect sits on top of Sacrifice.

The chain resolves in LIFO (Last-In, First-Out) order:

1. Rift Herald’s Deathknell resolves first: You are instructed to play a unit from your hand to your base. If you have no cards in your hand, this instruction cannot be followed and is ignored (Rule 359.3.e.11).
2. Sacrifice resolves second: You draw 2 cards and channel 1 rune.

Sequence of Events

1. Announcement: You declare you are playing Sacrifice.
2. Payment of Costs: You pay the additional cost to "kill a friendly [Mighty] unit" by choosing Rift Herald.
3. Trigger: The Rift Herald's Deathknell ability triggers. Per the rules regarding triggers resulting from the payment of costs, the trigger is placed on the chain on top of the Sacrifice spell.
4. Finalization: The chain is finalized in the following order (from bottom to top):
   • [Bottom] Sacrifice
   • [Top] Rift Herald's Deathknell
5. Resolution: The items resolve in LIFO order, starting with the Deathknell effect.

Note: This ruling is based on the interaction between Rule 357.2 and the LIFO resolution rules (Rule 340). I could not fully verify the citation of Rule 422.1.b against the retrieved rules.