Walking to the end of the green-lit corridor you can see that a lantern hangs on a long chain here, 10 feet off the ground. It seems that an everburning torch has been placed in it, and it is this torch that you have been seeing just after you entered the Cairn. The lantern itself is quite well-made and beautifully detailed.
(OOC: Picture below is of the indigo colored lamp, but otherwise it is the same.)
After a bit of searching the end of the corridor you uncover a 5-foot-diameter circle carved into the floor, directly below the hanging lantern.
(OOC: Meldain, what light source are you holding?)