Minecraft is a game, nice and balanced. Nothing is out of place, nothing is abnormal...not anymore
Omens of Darkness will change your world, put things where they shouldn't be, add things that shouldn't exist. For example, zombies will appear during the day, just to go up in flames. You can find skeletons without a bow. The already quite lands are even quiter as no music plays. These are all caused by strange entites.
Entites?
Spoiler
Entites are creatures that the human eye is unable to see. Entites will kill any animal they find, however, players will not be attacked. When an entity kills something it'll drop . .-. .-. --- .-. which is one of the many items you can use to communicate with the entites.
Progression
Spoiler
In order to communicate with the entites, you must first make a Block That Shouldn't Be, with cobblestone and rotten flesh. With it, you can make Blood Essence, with Blood Essence, you can make the Blood Sickle. Killing any mob with the Blood Sickle will drop Blood Faces, which can be used to make Darkness. With Darkness and Blood Essence, you can craft an eye. With cobblestone and Darkness, you can craft an altar. Put the eye in the altar...and the entities will help you from there.
