If my Ferrous Forerunner dies in combat with Rek'Sai on base, do I have to pay for Mechs to be accelerated?

Question

Accepted Answer

Yes, you must pay the Accelerate cost if you want the Mech tokens to enter the battlefield ready.

According to the RiftJudge FAQ (#8671, #9360), here is how this interaction works:

1.  **Triggering:** When Ferrous Forerunner dies, its **[Deathknell]** ability triggers and is placed on the chain.
2.  **Playing the Tokens:** When the Deathknell ability resolves, it instructs you to "play" two Mech unit tokens. 
3.  **Granting Accelerate:** Because these tokens are being played from an ability (not from your hand), **Rek'Sai, Breacher's** ability grants them **[Accelerate]**.
4.  **Optional Cost:** Accelerate is an optional additional cost (Rule 805.2). To have a unit enter the battlefield **ready**, you must pay the Accelerate cost—which is 1 Energy and 1 Power—as part of the process of playing the token.
5.  **Result:** 
    *   If you **pay** the cost for a token, it enters the board **ready**.
    *   If you **do not pay** the cost for a token, it enters the board **exhausted** (the default state for units entering play).

You pay this cost separately for each token, meaning you can choose to pay for both, one, or neither.