Location: Phantom Forest (After poison log)
Miss-able: No (Main Story Quest)
The source temple is located in the Northern Central area of the map, a waypoint should have been placed on your map through normal progression of the main storyline.
To enter the temple a puzzle must be solved in which 4 elemental switches must be pressed exactly one time, by having someone or something stand on them.
1. Starting at the south gate you should be able to see the AIR switch between two gates.
Either have a character with high telekinesis grab the Gate key that is sitting on a crate inside the gated area, or teleport a character over the edge to the inside. Either way pick up the key and unlock the gate and touch the AIR switch.
2. Directly inside the gated area, immediately after the AIR switch, there is a double sentinel surrounded by oil. There is a large circular switch that you can have a single character to stand on that should open a gate in the north-east located in the same oil/sentinel area. If that doesn't work you can always attack and break the gate since it is level 10. Inside you should find the earth/poison switch covered in lava protected by a sentinel. There are two teleportation scrolls in this area. You should be able to throw a barrel or similar item onto the switch to activate it, if that fails try teleporting something heavier (like a player) to activate the switch.
3. Head around the east side of the exterior to reach an area filled with switches and a tower of sorts in the middle of them. Take care not to step on most of the switches as they will spawn lava underneath you and kill you. You can aim a grenade/spell/attack at the ground to see which switches contain bombs beneath them. On the opposite end of this room you should see a switch, throw a teleport pyramid over near it and pull the switch. The traps should be disarmed and the gate should open. Continue around to the back of the temple and you should find a 3 pedestals. Search around the area for 3 statues - click and drag them onto their respective pedestals to spawn a teleport mirror. Go through the mirror and step on the switch.
4. Final switch. Stay up where the 3rd switch is located - you should see two switches connected together over looking a group of gates. Send one character down (preferably one that can sneak or invisible to get by a sentinel) on to the front of the first gate on the north side. Moving the switch between the first, second, and third positions and pulling the switch on the north side moves up and down the series of gates. Move the gates up and down and send the character on the ground through the gates, making sure to avoid the lava that spawn based on the sentinel and the position of the lever. Note, the lever position does also affect the final gate that the fireballs are firing through). Once past the fireballs, step on the last switch.
Finally, make your way back to the center of the Temple where the four switches have highlighted their respective runes and flip the switch next to the stone door. Ensuing is a fight against a fire demon. Once he has been dealt with you will be able to walk into the source temple, netting you this achievement.
Next Story Achievement: