Expeditions Rome: Divide and Conquer, choosing Zenobia or Theophilus, and the Temple of Apollon
Expeditions: Rome may have some hybrid strategy elements, but it's also a fully-fledged RPG. This means that alongside gearing your squad of praetorians and carrying out tactical battles with your legion, you'll also have to make some key decisions about who to trust and ally with, along with who to potentially assassinate. The Main Quest Divide and Conquer is one such quest, which is obtained early on during the Act 1 conquest of Asia Minor.
During the quest, you'll soon learn that you have lots of options available to you, and it's not clear which options lead to the best outcome. Here, we'll try to detail what's needed to continue the quest and the different potential ways you have of resolving the situation in Mysia.
Expeditions Rome: Divide and Conquer, choosing Zenobia or Theophilus, and the Temple of Apollon
Entering Mysia and the Temple of Apollon
Mysia is the first real region you'll come across after taking the garrison at Mytilene, which basically served at the tutorial of the game. The specific location in question is the Temple of Apollon, located just south of the Mysia fort. When you first enter the Temple, you'll be directed to the current leader, high priest Theophilus. Theophilus has thrown in his support behind King Mithridates, and it's your goal to help sway his favor towards the Romans or replace him with someone who is.
The first two merchants on the right-hand side can be spoken with for some quick dialogue. If you have Sickle Blade material, Boethus will offer to sell you schematics for the unique sword Harpe, which allows Attack of Opportunity to trigger twice.
The Sickle Blade is a random drop found in Greece in Act 1, so you may have to come back once you come across it. Boethus also sells weighted blades and javelin tools with which you can forge additional throwing knives and pilum, respectively. The other merchant, Alexios, can trade resources. If you have your legion garrisoned nearby in Mysia, you can extort him for 150 rations and 60 medicine (though at least you can offer to pay him for it too).
You'll run into Eritha and Xenoclea on the landing of the staircase on the way up to Theophilus. If you listen to their conversation, Eritha will then move over to a nearby courtyard with a statue. However, you won't be able to meaningfully interact with her quite yet.
There are a few other items you can get laying around here. In a chest underneath a large tree on a landing in the center of the area, you can get schematics for the unique Caltrops tactical item.
To the right of the staircase you can find a cracked floor that's easy to see if you have item highlights on (defaults to V key).
If you grab the Pickaxe weapon from near the temple on the left side of the area (see Temple Fire below), you can open the rubble and climb down to receive the Eurytus' Bow unique weapon.
Temple Fire and Olympian Side Quests
On the left side of the area, you can come across the side quest Temple Fire. This is a simple sidequest where you'll have to use water tactical items in battle to clear the fire hexes around the temple. If you have the Logos trait you can earn some extra rewards from the priest if you want. Upon clearing the fires, you'll complete the quest and earn the schematic for the Water Jug tactical item. You'll actually earn additional items in the form of some gear if you don't explicitly ask for a reward. You'll also likely earn the Adundans Cautela Non Nocet achievement here if you haven't earned it already, since you're not likely to take any damage.
Make sure that you open the chest with the Secret Missives behind the temple once you clear the fire. You can even find these before speaking to Theophilus.
Also make sure to grab the Pickaxe near the temple.
Near the beginning of the area you can also find an NPC standing on his own named Pigres. This will start the Olympian side quest. Once Syneros is with your party, Pigres will bring up memories from a botched Olympics event from several years ago where Syneros cheated to win an event against Pigres's grandfather.
You'll have a few options here ranging from letting Syneros lose to Pigres in a quick match, having Bestia take his place and defeat Pigres, or simply killing Pigres outright (which Syneros will greatly disapprove of). Whichever companion you send to fight Pigres will sustain an injury. Regardless of what you pick, this quest will continue later the next time you visit your camp.
Once you visit camp, you'll run into Diodoros, the man that Syneros cheated against so many years ago. Since Diodoros and Syneros are too old to wrestle anymore, you'll have to solve a series of riddles. The answers are usually pretty obvious, and having the Logos or Pathos traits will help as well. The selection of riddle is semi-random, so simply reload if you happen to get one wrong. Note that you simply have to eliminate the obvious wrong answer, so you effectively have a 2/3's chance at each riddle if you guess randomly. Upon success, Syneros will learn the Physician perk, which lets him treat burns, infection, and poison injuries. You'll also earn the achievement Citius Altius Fortius for completing the quest.
Speaking to Theophilus
With all that out of the way, it's finally time to talk to Theophilus. If you've already successfully taken the fort at Mysia, even if you haven't pacified the region, you'll be able to threaten the high priest with force and claim the temple as your own, but it won't do any good. Your various options here will please or anger your praetorians based on the options you choose, but you'll be forced to leave regardless.
On your way out of the temple, you'll run into Zenobia near the entrance of the area. She will ask you to effectively remove Theophilus from power. If you exhaust the dialogue options with Zenobia, you'll come to three possible ways to take care of Theophilus: Either poison him unexpectedly, lure him to a safe location where you can deal with him, or find a pressure point to provoke him into attacking first.
Find a poison for Theophilus
This is the least obvious solution for carrying out this quest, especially if you don't know where the poison can be found. You'll need to head to Troas Fishing Village, the same location as for the Whack The Mole quest.
Once in the village, you'll want to talk to the merchant named Theodoros. Simply ask about his "spices" and you can spend 400 denarii for some Bay Leaf Poison. Once you have this, you can return to the Temple of Apollon.
Near the top of the temple, you'll be able to find Rhianus. With a little bit of prodding, you'll be able to convince him to poison Theophilus with the Bay Leaves you bought. You can threaten him if you have the Ethos trait, or pay him 1000 denarii. Once this is done, you'll return to Zenobia for the next part of the quest..
Find a pressure point for Theophilus
This is where Eritha from earlier comes into play. If you haven't yet, you can listen to her conversation on the staircase and follow her to a small courtyard.
If you have the Pathos trait you may be able to get her to help you without having to kill her.
If you do kill her, you'll loot Eritha's Medallion, a unique amulet that restores 10% health for 10 turns, which is quite good.
If you end up showing this to Theophilus, he will attack you with his nearby guards. Simply defeat them all and return to Zenobia.
Search the Temple of Apollon for Theophilus' correspondence
If you haven't already, grab the Secret Missives from behind the temple from the Temple Fire quest. Once you have this, you'll be directed to talk to Syneros back at camp. Note that if you return to camp after starting the Olympian quest above, you'll run into Diodoros at the entrance.
Speak to Syneros when you're able, and he'll offer to forge a letter from Mithridates. Note that you can back out and state just to kill Theophilus, but this doesn't lock you out of going back to Syneros to forge the letters. Then you'll have to return to Zenobia. She will instruct you to meet them at a fake meeting location to the east of the temple.
Here, you'll run into both Zenobia and Theophilus and have to make a choice. If you side with Theophilus by either killing or apprehending Zenobia, you'll earn the Miserere Nobis achievement. You'll then be tasked to meet him back at the temple.
Kill the Pontic scouts and steal their uniforms
Regardless of the method chosen or who you end up siding with, you'll be tasked to steal uniforms in order to get into Mithridates' camp unnoticed. Head to the location on the map for another combat encounter. Having some units equipped with Water Jugs will help here, since the enemies do like to use fire on your units in this fight.
After the fight, loot the nearby boxes to find the uniforms. Here is where we also found the Sickle Blade material mentioned above, as well as the unique schematic for the Greek Fire Jar. You can either give the surviving soldier Gymnites 1000 denarii to keep quiet or simply kill him.
Infiltrate Mithridate's Warcamp and Return the Contract
Once you have the uniforms, you be directed to go to Mithridates' warcamp near Kyzikos to steal the contract between Mithridates and the temple. Here you'll also get an opportunity to flirt with Calida if you desire. In the camp, you'll see Mithridates and Scaevola conspiring against Rome, as well as introduce the mysterious figure Corvinus.
You'll be able to speak to Mithridates here as well if you wish. On the shelf inside the tent, you'll find the schematic for the Mithridate tactical item, as well as the quest item Mithridates' Contract.
Upon returning to the temple, Zenobia or Theophilus will thank you for your work. For completing the quest you'll get 140 XP, regardless of who you ended up siding with.