If Ferrous Forerunner dies while I have Rek'Sai Breacher on the field, can I Accelerate the two Mech tokens and how much should I pay?

Ruling: Yes, the Mech tokens created by Ferrous Forerunner gain Accelerate due to Rek'Sai Breacher's ability, and you may pay 1 Energy and 1 Power (any domain) for each token to have them enter the board ready.

Sequence:

• Ferrous Forerunner dies, triggering its Deathknell ability
• The Deathknell ability resolves, instructing you to play two Mech tokens
• As you play these tokens, Rek'Sai Breacher's ability grants them Accelerate (since they are played from somewhere other than your hand)
• You may choose to pay the Accelerate cost (1 Energy and 1 Power) for each token
• Any tokens for which you paid the cost enter the board ready instead of exhausted

Nuances:

• Tokens are considered "played" when an effect instructs you to play them, following all normal steps for playing a card
• Rek'Sai Breacher's ability applies to units played from anywhere other than hand, which includes tokens created by abilities
• You pay the Accelerate cost separately for each token and can choose to pay for one, both, or neither