Well,
as I understand the canyon looks like this.
There are two approaches for this:
Number one:
We do not require building a chimney to have the caves suck the smoke.
We just have to light a fire of the right size, and the channels/chambers within The Caves will circulate the smoke inside the caves thanks to the differences in temperature, as long as the smoke supply is continuous.
Number two:
We could actually build 2 or 3 “pipelines” with dirt using Telrith’s powers; the smoke would be directly fed into the caves by a structure similar to a chimney. I guess this is what Vicious means.
In both cases we don’t require an exit at the top of the caves or anywhere in particular, we just need to light the fire at the right time; that would be before dawn, when the sleep is heavy and the sun is about to rise, preventing combat without light.
The smoke will eventually seek an exit, but not before the sun rises and the environment of the whole region gets warmer. Such is the “chimney effect”.