I have some trouble understanding what is the issue with relighting the forge, granted, if the PCs also opt to leave the Hammer there as well, it would be even more troublesome.
Its not just a forge, its an evil altar to an evil giant god, if you have any good guys in the party they should probably not want to light it, as taking part in the ritual is an evil act in itself.
Saying that most of my party did not want to light it but the dwarf who has Agrimmosh managed to convince them.
On another note they are on their way to Janderhoff to talk to the Dwarves there about them sending a detachment to guard / use it. What would a Dwarven sky citadel pay as a reward for the party finding this , and letting them use the forge ?