Infestation is a survival experience, best played in hard mode.

When you first join the world, you will be given torches and bread. You will also notice a new bar at the bottom of your screen, which shows your Life Energy. Soulfeeders feed off of your Life Energy if you get too close to them, but Life Energy can be gained by killing the soulfeeders. You will also notice that it is always night.

To hide from soulfeeders, you can craft a bone mask using five bones and two string. When worn, you will not be able to see, but you can slip past soulfeeders unnoticed. However, if you wear it for too long around soulfeeders, it will break and you will no longer be invisible to the soulfeeders, leaving you vulnerable to attack.

It is not possible to harm soulfeeders directly, other than by using the Life Blade, which can be found in soulfeeder dungeons.

There is also a boss known as the Soulkeeper. It is a stronger version of the soulfeeders which can be summoned at the Soulkeeper shrine using 4 Life Blades. Killing it gives you the Amulet of Life, which when dropped on the ground creates a protective circle of healing and protection from the soulfeeders.

The final structure is called the soulfeeder burrow. It is an underground structure, marked only by a blue crystal on the surface. It is infested with soulfeeders.

If you run out of Life Energy, you will no longer be able to respawn. You lose 20 Life Energy whenever you die.


NOTE:

While the experience is playable without using the resource pack, I highly recommend using it. Everything makes more sense if you can see what you are meant to see.
Additionally, you will need to download Optifine to use the resource pack