. Spend this Power only to play units or gear.
ShowcaseCan Iron Revenant banish a unit or gear put into my trash as an additional cost to help pay that play's power cost?
No, because you pay the played card's energy and power costs before paying its additional cost by discarding a card or killing a unit.
You choose whether to pay an optional additional cost before determining the card's total cost.355.1.a For Atakhan and Commander Ledros, the variable discount from that choice is checked during total-cost determination, before payment begins.356.4.g During payment, you first pay the combined energy and power costs.357.1 You then pay non-standard costs, such as discarding a card or killing a unit; "in any order" applies only among those non-standard costs.357.2
During the energy and power payment step, you may activate Iron Revenant's [Reaction] ability to [Add] power.357.1.a At that point, the card that will enter your trash for the additional cost isn't there yet. Therefore, Iron Revenant can't banish a unit or gear discarded for Zed, From the Shadows to help pay Zed's power cost. It also can't banish a unit killed for Atakhan or Commander Ledros to help pay the power cost of the card being played.