Eluvian

An Eluvian (Elvish for "seeing glass") is a magical mirror.

Background
It is known to be a portal of some sort, and it can transmit the Taint. It was crafted by the elves of Arlathan to be used as portals for telecommunication and rapid transportation between their cities, using a type of magic different from that of the modern Circle of Magi or even the Tevinter Imperium.

During the fall of Arlathan, many Eluvians were destroyed, while those that survived the conflict seemingly stopped functioning. An unknown number of Eluvians survived to the present day, scattered across Thedas in forgotten or well hidden locations.

The Tevinter magisters attempted to unlock the secrets of the Eluvians, but all they could use them for was long-distance communication. Some Dalish have also attempted to rediscover and reactivate the Eluvian network to no avail.

Dragon Age: Origins
During the Dalish Elf Origin, Tamlen and a Dalish Elf discover an Eluvian. Tamlen claims to see scenes from an underground city through the mirror and proceeds to touch it. This causes a flash of light, which causes the Warden to lose consciousness for two days. After this incident, Tamlen disappears.

Trivia

 * Duncan misidentifies Tevinter as the source of Eluvians.