There are several kinds of bosses in Dream World.

At the end of each area there is a boss that must be defeated to advance to the next area.

From level 51 onwards you may encounter the previous area bosses that have been possessed by demons.

After level 51, the town leaders can be fought as bosses.

The Demon Gates, Alien Portal, Alien Invasion and Isendel have bosses that all guild members or all players on the server can fight together.