This section is about the obelisks and where they are located. I will tell you what stage they are in and a detailed description on how to locate it.
Dagger Peak: Take the lower path at the start, follow it around until you see two generators, between these is a rock-like covering. Just beyond this is a split in the road with 3 ways to go, go to top-right. At the next split, take the upper path. At the top is an obelisk just sitting nicely in the corner.
Cliffs of Desolation: Take the upper path, it leads straight the third obelisk.
Dungeon of Torment: At the stairs, go down not straight, hit a hidden wall panel at the base to reveal a switch. Get the money in the corner to hit a second switch, go back up, take the stairs you skipped before. At the base of these stairs, go down to hit another switch. Go back to the other stairs and go down those again, cross the acid pool at the bottom this time. Hit all the switches you see, cross over using the 2 player switch and hit the switch there too. Go down from that switch to hit a 3rd switch, then go up through the gate and turn left, hit the switch on the bridge and continue, you should be back and the 2 sets of stairs. Cross the room and go up the stairs, when you see two sets of stairs, take the upper route. Turn right, hit the switch continue to the next room, here, cross the bridge. Turn left and go up the stairs. Hit the switch you see on the right, go back to the bridge, hit the switch there now go where it points to get the Obelisk.
Haunted Cemetery: Hit the switch under the barrel on the left, go up through the gates, hit the switch under the barrel in the middle of the back of the group of gravestones, this will open the gate to the next area at the top, go there. The switches around the gravestones just raise some graves to give you items. Follow this path until you reach another set of gravestones, the switch you need to hit is in the rear-middle again, behind a tree. There is a switch is the small house in the right-hand part of this area which raises some graves to give more items. Go to the left when you are ready. Follow this path until you get stopped, on the left is a switch which opens a path to the right, go there. Hitting the switch under the barrel in here opens the door that was locked before, go through that door. Here, shoot the doors on either side of you to open up items and switches. Continue on the path. Eventually you'll end up in a huge open grass area, go as far left as possible, there will be another house, BEHIND it, not in it, is the Obelisk.