1 - Onyx Keyhole

2 - Key Trap
If you pick up both keys you will fall into a trap. Pick up one at a time.
Contains: Onyx Key, Skull Key
Get the Onyx Key first, lower the ladder that can be reached at the Valve Station where the blue teleporters blink around. This way you can get inside without the use of the Onyx key.

3 - Remove the cover plate and press the button.
Spawns an Excsymyr at 4.

4 - Spawns an Excsymyr by the secret button at 3.

5 - Attack Minion

6 - Skull Keyhole
Opens a teleporter to your left when you use the skull key. Teleporting you into a room where you lack space to do anything. Simply shoot a fireball into the mirror and destroy a table, making it possible to move again.

7 - Activating the Void gate.
Take the reflectors from the Skull key room and face them in the way the fireballs will end up on the power crystal in the center of the room.
Then you have to make sure everything is working, the furnace, the valves, etc.
Tip: The furnace dies down because theres a guard minion spawning on top of it. Making it hard for the rockmen to keep the thing lit. To prevent this, Leave some guard minions of your own to shoot theirs down.

All 3 of the blasts have to hit the power crystal at the same time for the void to open, it may take some tweaking here and there. But eventually the Gate to the Void will open. Make sure you are properly prepared for the final bossfight.