Agrael is being hunted by Godric, at the command of Queen Isabel. You'll have about two turns to begin running as fast as you can to the end of the level, however you'll also need to pick up reinforcements for the battle against Ethriel.
Make note that The Cultist campaign is much trickier than The Queen campaign; you'll have to avoid some things, while going out of your way for others. Always be sure to pick up reinforcements when you can, and gather gold. Artifacts, other resources, and special objects are just there to get in your way.
A fresh start
From the beginning you should run straight south. As the path turns eastward, make sure you grab the Witch Hut to pick up Logistics (10% move speed increase). Take to the hills, and continue south off of the stone path until you reach a pack of Griffins next to a Redwood Treetop. For the battle against the Griffins, simply wait for them to come to you. Blast them with everything you have, then charge them with your Cerberii when they get close enough to attack you.
Use the Redwood to see around you. Head south and try to grab the gold pile next to the Horned Demons. The Demons will aggro, and you'll be able to have them join your party. Grab the gold (ignore the wood), and head northeast and try attacking the familiars. They, too, will join your party.
Head southeast until you can see the garrison. Closeby, to the northeast, you'll see a grassy path. Follow it eastward, fight the group of monsters, and continue.
The Red Keymaster
If you go too far southeast from here, where you can see the one way monolith exit and the Red Gate, a cutscene where Agrael talks about escaping into the tunnels to Sheogh takes place.
You'll have to talk to the Red Keymaster (in the tent to the northeast) before you can open the pathway that the Red Gate blocks.
Run north, kill the mob, grab the spell power upgrade, then try and get the Cerberii to the northwest to join you. Head east along the road, then south and fight the two groups of monsters. Speak with the Red Keymaster, then use the one way monolith to teleport southwest to the gate (if Godric is closeby, you can use the monolith gate to confuse the AI and buy you a lot of time).
Inspect the gate, and open it. Head down into the subterranean area. Kill the Plague Zombies to the south east, and gather the two gold piles and the treasure chest (choosing gold).
Follow the path northwest now, and make sure you don't fight any enemies, but pick up all the enemies you can along the way. Stop at each recruiting place, and remember to avoid the fire elementals on the other side of the curve (near the top of the map). Also note that the Cerberii at the top of the curve will not join your group, but will flee instead.
Follow the path to the southwest until you reach the stairwell. You should save your game now, as the final battle of the level will take place. If your forces aren't strong enough, you might have to retry the entire level to mass enough forces.
As you head up the stairwell, Ethriel will come charging down the path at you. He'll stop, a cutscene will take place showing some intimidating forces, then attack Agrael.
Etheriel has 1xArchdevil, 2xPit Lord, 15xSuccubus, 30xCerberus, 75xFamiliar, 50xHorned Demon. If you try rushing into this battle without enough troops, you're sure to be dead. You're going to rely heavily on luck and your group of Cerberii. Place your Cerberii in the center of your line before battle, then charge them across the field so they can attack both the enemy Cerberii and the Pit Lords at the same time. If all goes well, the Pit Lords will be dead, and most of the Cerberii will be gone too.
Use your pit fiends to try and maximize damage, however they'll probably do best with just using Meteor across 2-3 enemy groups. Do not have your Succubi attack the enemy Succubi, or you'll only take extra damage. Have Agrael attack them, and wait for a melee unit to get close enough to stop them from using their ranged attacks.
Agrael will level up at least one time after this battle, so make sure to choose some of your choices before the game tries to move on.
A cutscene where Ethriel spills the beans on what's been going on happens, and he ends up being fried by the Sovereign.