Rifts are deep ravines created when another Nether Gate rips its way into the world.
The power of the Gate tears the world apart as it enters the world and often leaves a ravine in its wake.
The Taint of the Black Gates hands still in the air of these ravines, like a dry mist which sucks the life from you.
These rifts are avoided by many, except those who hail the darkness that created them, who can often find sanctuary within or near them from the prying eyes of those who would want them dead, rather than for them to continue to serve their Lord Herobrine.
Rifts were created when Nether Gates enter our world, which the original source is from when the Magical Order tampered with Dark magic in its attempts to learn more ofn the arcane arts, but it made a mistake and from that the gates entered the world.