This is a game about being afraid of the dark. It's kind of hard to make a 2D overhead game scary, but I think I can definitely ratchet up some tension.
This is a game where you play a group of researchers exploring the forgotten tomb of an ancient mummy. You play them simultaneously in real time and they each have unique abilities - not unlike Commandos or Desperadoes. The basic structure of each level is that you have to combine these unique abilities to overcome obstacles and reach the stairs leading to the next level. I won't bother going into too much detail about that stuff, since this is about the darkness.
Basically, your worst enemy is the darkness. Darkness isn't, itself, dangerous, but within the darkness lurks enemy horrors. Each level has a specific number of enemies and they can move anywhere in the darkness, but won't step into the light except in a few specific circumstances (to be detailed in a moment). Generally speaking, if you are in the light, you are safe. Much of the puzzle of the game involves changing the lighting conditions to make sure that all characters are in the light at all times.
Like I said, each character has a few unique abilities and this extends to their ability to create light. All of them are capable of carrying torches, which create a light radius around the character. Torches grow dimmer over time before extinguishing completely. For unique abilities, one character has a flashlight, which is a directional light source. Rather than a light radius, it emits light in a specific direction (leaving your sides and back in the darkness). Another character has a lighter, used primarily to light found torches, but also emits a dim one tile radius around the character. One character has a lantern, which contains fuel and can go a long time, but that fuel can be poured on torches to make them last longer. The final character has a night vision camera - it doesn't illuminate the area such that the creatures won't attack, but does allow you to see them coming. Also, there are some areas which open flames can not be used (maybe high methane concentration or something) and in general, you don't want to waste your limited light sources if you don't have to.
Within the levels, you can find things which help you maintain light. Torches on the walls can be lit and even taken. There are fire pits which can be lit. Mirrors can be pushed and rotated to reflect light around corners. Kegs of powder will explode when lit, but you can also leave a trail of powder and light it to create a quick burning light source (perhaps one that leads to a big boom in the end).
Normally, the creatures in the darkness are unknown (the shot above is lightened for illustrative purposes - normally it would be completely black in darkness). However, they are attracted to blood and have a good sense of smell. If you wander too close to the dimming darkness, there is a possibility that they will leap out and attack (assuming they are there waiting for you). The darker the square, the more danger you are in. Also, the larger the concentration of them there are, the more brave they become. If there are three or four in the darkness, they may be brave enough to work into progressively lighter areas. They may also brave the light for a quick jump between two dark areas.
If you light up a tile with a creature in it (and they are alone), you will catch a quick glimpse of them as they run back into the darkness. In this way, you can heard the creatures by blocking exits with torches - but again, group too many together and they get brave. However, if you end up removing all darkness and the creature is stuck in the light, it becomes fierce and will attack on sight, so you don't ever want to truly expose one, even by accident.
Being in the darkness is not a death sentence in itself. They have to find you first. It should be possible to leave some meat in one area of the level to distract them, then make a run through the darkness. The creatures navigate by smell (the are partially blind) so if you stand still in the darkness, aren't bleeding, and they don't walk close enough, it is possible the creatures will not even notice you are there. Of course, unless you have the night vision camera, knowing exactly where they are is next to impossible.