These objects provide magical exits to rooms. Some are permanent fixtures in certain areas or may be temporarily created as part of certain ticket quests. Others are temporary creations of characters who cast 'portal'.
Portals may potentially "drain" certain small random amounts of hit points, mana points, and/or move points from characters who travel through them. Each individual portal has a certain value that determines this, making its chances of "portal drain" on characters either 0%, completely random, or 100%. characters who create portals usually create this second type, although sometimes either a "critical failure" in their spell may result in this third type or else they may get lucky and produce a "perfect portal" of this first type.
Although traveling via portal can never kill characters directly, it can be indirectly deadly for relatively-low-level characters who, for example, may arrive in a dangerous area with zero hit points; by contrast, relatively-high-level characters with plenty of points have much less to fear and may hardly ever notice any drain. Mages may cast 'astral shield' on characters to prevent them from experiencing any "portal drain" during transit.
See also Nexuses.