Explaining how to navigate the Dark Side with just the small light of an orb is probably going to confuse you more than anything. For that reason, I'm simply providing a darkness-free map, the same thing you'll see in the game without the black covering everything but the small circle around your player. Green circles mark treasure chests and BOSS is self explanatory, sort of. It's not self explanatory that this is the final boss. The next section details that battle.
A few notes on some of the enemies you may encounter on the dark side: first Shadows. Shadows can turn you into a shadow which will mean on that players turn he or she will attack another party member. Always heal ASAP when one or more people becomes a shadow. Awoken -- these skeleton enemies possess one of the most powerful spells in the game -- Darkness3. Try to stun these guys early on so they can't cast it, and also keep everyone cured well up in case they manage to do so anyway. Another notable enemy is the Seeker -- they attack in packs of 4 and prey on the weakest player in your party. Luckily Seekers are easy to destroy. Finally, Devils can hurt you bad. They have a Touch of Death spell which is exactly like the Girl's Kiss of Death spell (it will kill you instantly).
Final area map: