First, go to the Plains temple with teleport stone or walk to it. The location of the plains temple is just north from Severin's farm. You will find a couple lizardmen and a Saurian leader. There are five Saurian leaders in total (One on the way from the main entrance of the Harbour Town to Stan, one near the rear exit near Leto, one that was attacking Jasmin's hut in Chapter 3, one in front of the plains temple, and one near the waterfall in the East Coast.) Kill them to complete the Quest of killing all Saurian leaders in Chapter 4.
Kill the leaders and open the chest in the entrance of the Plains temple if you didn't do so, then kill a war cricket and get inside. You will see 2 Scorpions. Kill them and proceed further into the temple. Choose the path on the left but don't go too far. There is a trap that will crush you into ashes by a huge rock. Use telekinesis to pull the lever and deactivate the trap. Kill a few elite warriors and proceed through the left and kill the Ashbeast you see. Jump across the chasm and climb up to reach a room that contains a High Priest. Use the scroll Destroy Magical Force Field to break the red magic barrier.
Enter and take things in the room. You will see a sarcophagus in the centre of the room and two bottles of potions. They are Life Potion (Max HP +10)
and Experience Potion (+100 XP). There are also two scroll, light healing and conjure skeleton. Open the sarcophagus and it comes out a skeleton warrior.
Get back to the main room (The place that you have to deactivate the trap to go through) and turn right this time.
Break through the Blue barrier and break through the stone door.
Go to the center of the room and turn around, facing the stone door that you broke through. Look up and you will see the priest's bust and take it by telekinesis.
Place the bust on the altar to open the door. Open the sarcophagus to retrieve the Titan Armour piece, a Titan Staff, 145 Gold coins, 3 Lumps of Gold, 2 silver armour plates, a torch and a light spell scroll.
Get out of the temple by teleport or walking out of it.