To get started, You should find an occult structure. These are usually large, looming structures with a parkour course which you have to complete in order to get its loot. The loot in them can be very useful, such as Bewitchment Orbs.

Bewitchment orbs can only be found inside of these structures, they are used to bewitch items such as Sacrificial daggers, or the gloomy hood. Bewitched items have special abilities, and sometimes a curse. 

They can be found in 4 states.

Small, basic, fate, and soul.

Small orbs give a tiny boost that doesn't really do that much difference.

Basic ones, give a somewhat good ability that can be powerful if used well.

Fate orbs have a really useful ability, however they have a bound curse that comes with them, meaning if you have it on a tool, then while that tool is in your inventory, then you also get a curse.

Soul orbs aren't for items. but for players. They take up your inventory space in return for an ability while they're in your inventory, they can be crafted by using a part of a mob, and soul bewitchment orb fragments.

Bewitched items change their apperance:

Occult also offers many new decorative blocks.

However, every good mod has to have a threat..

The bedbug is a brand new mob that appears when you try to sleep in an unlit bed, they are characterized by tentacles which form under beds if they're not in enough light.

The bedbug itself isn't a threat, however what it does, is. The bed bug has an ability called bug vibration, which works like the warden, however instead of it getting attracted to mobs, mobs get attracted to it. So all mobs will come towards the bedbug if not killed fast enough. The bedbug is unpredictable and will attract hordes of mobs near you

There's also many other beings, however I'll leave that to you to find out :)