In order to enter the Darkworld, you'll require the flashlight.

You'll be able to find this in the Androgen Assault level, follow the wall along to your right as soon as you spawn.



This will then bring you into this room with several open windows, jump through these onto the platform ahead with two wooden planks.



You should then see this enemy here, who is wielding the flashlight we need.



The flashlight is more of a tool than a weapon and can't be used to fight enemies, it is advisable that you kill him and take his weapon as you leave the level in order to unlock it for your loadout.

The access to the secret level is located within the level Paradise, before you start make sure you have the flashlight equipped in one of your weapon slots.

As soon as you enter the level you will need to enter the green house to the left.



Further on inside the house, you'll come across an entrance to the basement.
(There is also an alternative entrance to the basement over the fence around the side of the house.)

Head inside there.



In the basement, you'll find a door that looks exactly like this. To gain pass through it, you'll need a divine link established to your character - you can tell whether you have this active from the white and red border around your screen.



If you do not have a divine link, you can get it by beating any incomplete level on Punishment mode or by using the Divine Link room around the starting area in the level Archon Grid.

Once you've opened the door to the catacombs, head to your left as soon as you reach the end of the hallway.



Follow the wall along to the left here, and head towards the next room you see by your left.



You will then come into a room with a giant hole in the middle, jump down here.



The path here should be straightforward enough to lead you directly into a tunnel that looks like this, carry on forward.



At the end of the tunnel, you'll come across a bridge that will lead you into a room with a painting of the Darkworld on it.

Jump into this painting to unlock the Darkworld.

When brought back to the title screen, you should see the Darkworld unlocked.



Be sure to bring a light.