Kill a Wind Keeper to get a Wind Blessing. Lasts for 20 minutes.
Kill a Fire Keeper to get a Fire Blessing. Lasts for 20 minutes.
Kill an Earth Keeper to get an Earth Blessing. Lasts for 20 minutes.
Vanquish the Elemental Guardian.
Proceed to the ritual chamber upstairs.
Use the item from the last mission "Arbeh's Bane". Arbeh is invulnerable at first. You need to eliminate the worshipers. Using the artifact will turn one of them into a bug which can be killed now. Click "Arbeh's Bane" again and another worshiper turns into a bug... Go on like that till you killed all of them. It might help pulling em down the stairs to avoid attacks of Arbeh. Now use the artifact once again on Arbeh and you can do damage on him. If he puts shields up and might be invulnerable again, use the artifact on him. This will lessen the strength of the shields or even remove them. There is a bug that Arbeh's health bar might turn yellow and seems to be full again. Don't get irritated. He still takes damage as you can see by his life points in the yellow bar.