A metaphysical barrier exists between the physical Thedas and the Fade. Neither spirits nor mortal beings can easily pass physically through the Veil, but the consciousnesses of most mortal dreamers and all mages are a different matter (though the average dreamer is unlikely to ever be aware of such 'travels'). The only ones able to physically pass trough the Veil are the Dreamers (also called "Somniari" by the Elves), rare and powerful mages capable of entering the Fade with their physical body at will.
The Veil is not an object; it cannot be touched or seen. It is a metaphor used by mortal scholars to explain the interactions between the two different realities.
The Veil tends to be weaker in places that feature (or have experienced) extensive death or use of magic; areas that can be seen or felt where the Veil is thin are usually referred to as 'Tears' (as in 'tear fabric', not 'cry tears'). 'Mending' these Tears (i.e. strengthening the Veil) is extremely difficult (when not impossible), especially for those unfamiliar with the area. For example, in Soldier's Peak, either Sophia Dryden or Avernus can accomplish such a mending (Avernus being the likely source of the tear, and Sophia being a powerful abomination who has remained in the vicinity for centuries), but it's doubtful they would be so effectual in the Brecilian Forest. The Warden has opportunities to mend Veil Tears in both Awakening and Witch Hunt.
The rare plant Felandaris (or "demon weed") grows only in places where the Veil is thin.
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