These objects provide magical two-way connections between 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 'nexus'.
Nexuses may potentially "drain" certain small random amounts of hit points, mana points, and/or move points from characters who travel through them. Each individual nexus has a certain value that determines this, making its chances of "portal drain" on characters either 0%, completely random, or 100%.
Although traveling via nexus 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 Portals.