Yep, the brightness in game will not brighten on maps that do not allow for it. If you can brighten your monitor settings, try that, otherwise try straight from your video card. If you have Nvidia, there should be a control panel from them that lets you adjust the brightness or gamma for your whole PC. If you are on Windows, you can get to it by Control Panel --> Display --> [Tab] Settings --> Advanced. For Linux, I've installed the drivers from Nvidia's website to get the control panel, but not sure how to do it in another way. ATI should have something similar with the drivers installed. On a laptop, I have seen some adjustments for their basic video driver under the Display Settings, too.
Once you get through the Death Realm once, it is much easier the next 250 times. And yep, the DR has some good reasons to go down there, so it isn't always a big punishment, can be fun.