There are currently 5 types of portals. Portal will have a chance of randomly appearing when a monster is slain. Only 1 portal of it's type can be open at once and they will last 3 hours. After 3 hours, if the guardian within is not slain, the portal will close and everyone within will be ejected. You will be able to tell an average of the remaining time by clicking on the portal. Guardians will have a chance of dropping a rare piece of clothing (a whole set is 5 pieces). Each guardians will drop its own outfit.