Lyric discussion by OnDistantShores 

Cover art for Titan lyrics by Brave Saint Saturn

It's about sin. The "darkness inside", his sinful nature, is destroying him, bringing him down, keeping him from the light, and eating him away like a cancer.

He doesnt explain much in the way of how this practically affects him, but its kind of a look at the spiritual warfare going on over our souls. This is evident in reference to the spiritual realm in "behind what I know and see" and constant use of "light" and "darkness".

Good song.