Instructions
Make your way to the third tower in the Black Throne section. Load an earlier save, if necessary, since you can lock yourself out of access to the locked room at the very top if you don't go off the beaten path in this section. Make sure that the first and second tower sections are cleared.
Enter the large, circular room in the center of the tower, and then activate the portal directly by the crystals. Enter this portal, and then bypass the rotating pillar.
Bomb the red crystals in the room, and then return to the top of the stairs. Drop a portal here that will open over the activated portal in the pillar, and then jump through these portals in order to end up on a ledge with the Beholder Key.
Exit the area and go back to the spike pit room in the main area of the tower. Wait for the moving pillars to move low enough to reach you, and then ride the pillars all the way up to the top, where you'll find a blue, locked door. Open the door with the Beholder Key to fight a few enemies and open a chest with an Abyssal Shard in it.