Soul Fuel

The engine at the heart of every infernal war machine has a furnace fueled by soul coins. Among the vehicle’s helm controls is a narrow slot into which soul coins can be fed. soul coins placed in this slot tumble into the furnace, which is two size categories smaller than the vehicle containing it.
An infernal war machine’s furnace consumes a soul coin instantly, expending all the coin’s remaining charges at once and destroying the coin in the process. The soul trapped in the coin becomes trapped in the furnace instead, powering the infernal war machine for a duration determined by how many charges the soul coin had when it was consumed: 1 charge, 24 hours; 2 charges, 48 hours; 3 charges, 72 hours. If it’s still trapped in the furnace when this duration ends, the soul is destroyed. Not even divine intervention can restore a soul destroyed in this manner.
Regardless of its size, a furnace can hold any number of souls, their screams of anguish audible out to a range of 60 feet. Each new soul fed to the furnace adds more fuel to the vehicle, allowing it to run longer.