Sometimes creatures will execute a raid on a nearby city, dungeon or open area. While some raids are announced via messages broadcast to all players online at the moment, others happen with no warning at all. In most raids, monsters disappear after a few hours, but some others (such as Gaz'haragoth) will stay until next Server Save.
If you have completed an area of the Cyclopedia Map through the Measuring Tibia Quest, you'll see a warning Iminent Raid Icon on that area that a raid will start soon. Fifteen minutes before the raid begins, the warning will be displayed on the raid center instead.