Equip it, wait till one spawns, and unequip. The nightmare creature will not deaggro/despawn. Kill it, wear the amulet again. A much easier way is to be in the ruins in nightmare phase. Stay around the night lights, and wait till they spawn. DON'T STOP RUNNING untill the end of nightmare phase, after which they all fall and die, giving you about 15 nightmare fuel in less than half a day.