Official Riftbound ruling

What happens if I play Atakhan from my deck with Rek’Sai, Breacher on the board? Can I pay the Accelerate cost and kill Rek’Sai for the Atakhan additional cost? Will it enter ready?

Yes, as long as you pay the Accelerate cost, Atakhan will enter ready, even if he kills Rek’Sai for his additional cost. This is because Accelerate creates a delayed replacement effect that causes Atakhan to enter ready. Although he lacks the Accelerate keyword when he resolves, he still has the delayed replacement effect that Accelerate generates.

Riot's question and answer, word for word, from theUnleashed Rules FAQ and Clarifications

The cards

  • Atakhan
    Atakhan

    Unit

    You may kill a friendly unit as an additional cost to play me. If you do, I cost 1 Energy1 Energy less for each Energy it costs and Order RuneOrder Rune less for each Power it costs.

    Ganking (I can move from battlefield to battlefield.)

    When I attack, the defender must kill one of their units here.

  • Rek'Sai, Breacher
    Rek'Sai, Breacher

    Unit

    Accelerate (You may pay 1 Energy1 EnergyFury RuneFury Rune as an additional cost to have me enter ready.)

    Assault (+1 MightMight while I'm an attacker.)

    Friendly units played from anywhere other than a player's hand have Accelerate.