Don't think about it too hard, leave it alone, keep playing, don't leave, don't look at me

yeah, you're fucking cooked :(

You haven't thought about much other than basic survival in a while; eating, sleeping, killing monsters and mining for materials, but one day... You look at your base. It sucks, just some wood, logs, and a cobblestone slab roof. You need to do something about this, so you start building. After a while you've gotten everything you need but wool, so you take a sheep from the area, you make your room- It looks grand, rich black woolen carpeting and royal sheets of black wool on a comfy bed... One day you go outside to shear your pet sheep once more, but it's not acting itself.

The Threat:
A being that seeks to smite the only builder left, it was sealed away centuries ago by that civilization, but after so many years of absence that seal has broken, and now it has it's sights trained on you.

It can create and alter mobs and items, passive mobs change in appearance and behavior and the environment subtly changes around you. Infected mobs become useless, sheep no longer produce wool and grass they eat doesn't return, cow's milk spoils and turns into black ichor that can't be consumed, pigs are... Already useless. Mobs like zombies and skeletons become more aggressive, honing in on the player's location and stalking them with intelligence.

It can affect crops, too, they wither and turn black, turning to ash at the slightest touch and becoming inedible. The only way you can survive is by eating the rotten flesh dropped by zombies and the strange gifts found in your chest and furnaces.

Eventually, it will show up to try and end you itself, it can't be reasoned with, but it's still inevitably interested in you, so survival is... Vaguely possible.