To get started, you need to find one of the shaded structures that are spread throughout the world. One of which is a shaded tower as shown in the image below:

These shaded towers contain chests full of loot left by something wicked. If you manage to get your hands on one, you may find useful things for building blocks, items and even weapons. One of these things are the shaded bricks and seeking lamp which are good for setting the mood to something more spooky.

If lucky, one may find one of the demonic beings that came with all of this loot.

Occultores are shady humanoids made of twisted fiber with a singular wide eye. They are aggressive towards any player they see and due to their big eye they are able to spot players further than most monsters. If they manage to damage a player they will go into a phantom phase for a short amount of time causing them to become harder to spot and more deadly by making them faster and regenerate health if any was lost.

If slain, they will drop a couple of twisted fibers and possibly a accursed potion.

These twisted fibers can be put together with a watchlith to create shadow fiber, a very important component in dark magic crafting. 

Now a lot of crafting opportunities are unlocked.

From a shaded tower chest you may also find an occult smithing template, with this item you can now obtain a very powerful weapon known as the sacrificial dagger. This dagger has dark magic stored within.

And lastly for the current version of the occult, flags. Flags are a new decorative block which you can give a preset pattern to.