For this quest, you must travel to the Temple of the Sun God shown below.
Tier 1: Clear the temple courtyard of cultists
The first part of this quest is fairly self explanatory. There are cultists in front of the temple. You need to make it so that there aren’t quite so many. Kill the required 12 to more onto Tier 2 of the quest.
Tier 2: Find a way into the Temple & Proceed to the Ritual Chamber Upstairs
In order to enter the temple, you must defear the four elemental keepers that guard it and obtain a buff from slaying each of them. The four keepers are Earth, Wind, Fire, and Water. Two are on the upper platform, and two on the lower platform. The order in which you slay them does not matter, as the buff you receive lasts for 20 minutes. Once you have all four buffs on yourself, you will be allowed entry into the temple through the bottom-central doorway.
Tier 3: Vanquish the Elemental Guardian
When inside, you will be faced with an Elemental Guardian. It may be a good idea to kill the two bugs on either side of the room first before continuing, because you may need the extra space to safely kite him around the place. As his HP drops, he will spawn little guardians of water, fire, earth, and air. The good news is that they have very little hp, and all of their attacks are ground based. This means they will never auto attack you, and the only way they will hurt you is if you stand in the white spaces that spawn under them when they begin charging an attack. Just keep moving around the room either AoEing everyone down, or focusing on the main guy. Eventually they will all die and you will be able to proceed to the chamber upstairs.
Tier 4: Defeat Arbeh
For this fight, you need to use the item that you were given at the very start of the mission. It can be found in your inventory. Every time you click it, one of the worshipers surrounding Arbeh will turn into a bug and become attack-able. Kill it and use the item again. Keep this going until all the followers are dead. Then use the item every time the boss gets a shield up / stops taking damage. He should be easy to kill. Just make sure to stay out of the stuff he leaves on the floor.