The concealment scroll is some very cool magic. It allows one person to turn a whole team invisible. This allows them to kill grouped mobs one at a time, such as in Skaldor's lair. It even has theoretical playerkilling uses, since a team of assassins could surround someone and backstab them to death.
Killing Skaldor with concealment requires some discipline, since people have to attack one target at a time and avoid accidentally hitting others, especially when the skeletons see gold or weapons on the ground and rush to pick them up.