The phrase "up to one" was inadvertently omitted from Mass Mutiny's rules text. The card has received errata to correct this.
2012-06-01 • wotc
You can cast Mass Mutiny even if an opponent doesn't control any creatures. You simply won't choose a target for that opponent.
2012-06-01 • wotc
Mass Mutiny can target untapped creatures.
2012-06-01 • wotc
Gaining control of a creature doesn't cause you to gain control of any Auras or Equipment attached to it. However, if you gain control of a creature enchanted by an Aura with umbra armor and that creature would be destroyed that turn, instead the Aura will be destroyed and the creature will survive.
2012-06-01 • wotc