Continue to the deep to find a chained, giant Gorecyclops. You may want to save your game first. The giant cyclops is a very formidable foe so avoid damaging it. It will stay chained and will not notice you until you touch it. Should you decide to fight it, you have to be a ranged-class at least and stay on higher ground as you deal damage on its head. There's also an opening to the southwest leading to a sewer. Avoid touching the toxic water to avoid getting poisoned. There are four chests here and several leapworms.
After taking the first set of stairs, check the path to the northwest as it leads to a cavern with two chests.
Continue upstairs and take out the skeleton mages and gargoyles. Follow the path until you reach the topmost level. You'll find a chest and a corpse with the Void Key. Loot them and return back to Duskmoon Tower and open the gate in the middle.
AWESOME TIP!!
Past the chained Gorecyclops, through the path to the toxic red water, next to the third chest, drop down to a ledge to the left of the toxic falls to find a dead warrior in a cove with many rift stone clusters.