You and your fellow Tibia players will face off against the toughest creatures that you can find in Tibia, both old and new ones. Even some old classic bosses, such as Ferumbras (oh, you will see a lot of him), Orshabaal, Abyssador and Morgaroth will join the fight.
If you're lucky you may also see (and hopefully slay) some bosses that have, so far, only appeared on test servers, such as The Abomination, Verminor or Apocalypse.
Monster rampages are a lot of fun, be prepared to die a lot!