As said before , this guide is to reform the Roman Empire. It is not a speedrun guide, it is not absolute, but it should help you.

We will reform the Roman Empire as an Orthodox character, but you can do it as a catholic, miaphysite, iconoclast, hellenic or even as a messalian ( I did and it was super fun by the way). We won't be using big optimisation but I can tell you this: if you want to stay christian, the quickest way is to go cathar (or any catholic heresies) so you can holy war your way through Europe.

Now let's start.

When you start as Byzantium, a lot of laws are enacted for you, such as vice-duchies and vice-royalties. For some reasons, people tend to not like vice-duchies and kingdom but I'm telling you : KEEP THEM, IT WILL HELP YOU. The reason it helps you is that each that the vice-duke or vice-roy dies, the title goes back to you and you grant it back, you get an opinion boost and it will help you keep the realm stable. The same goes for kingdoms. It requires some micro, but the game should stop when you receive a vice-duchy, just grant it back. Even better, you can give it to another family to keep vassals weaker but I didn't bother, so you can ignore that if you want.
At the start, you are Iconoclast. It's a sh*t religion, especially because you have the Hagia Sophia and Icoconast can't use it. If you want however, you can stay Iconoclast, but Constantinople will be weaker because you won't be able to use its wonder.

Something you should know is that as the Byzantine Emperor, the patriarch is your vassal, so you will be able to excommunicate every dukes and lower who are dierctly your vassals. And a excommunicaed vassal is elligable to constant arrestation. Use this to your advantage. We will later in this guide.

Also, the Byzantine Empire is a Imperial Government. It means that you can keep cities as your holding. They are super useful because they give you a lot of money and quite a bit of troops and tech points. Keep them when you can!

You should start the game with either Rome or Constantinople as your preferred capital for the futur. I personnaly advise to keep Constantinople because it starts very strong with the Hadrian's Wall (or whatever they are called) and 4 cities + 1 castle as sub-holdings. It makes for a very strong capital that can give you easily 10K troops.
If you were to choose Rome (or Jerusalem which is pretty fun, I've done it and it was worth it), plan to conquer it ASAP and spread technologie in the county. Keep Constantinople meanwhile to still have some personal levies. To be honest, you should keep Constantinople all game either way because it is a very strong county.

Your character
When you start the game, pick your focus. I usually pick War or Intrigue, but whatever suits your gameplay, it's not that important. However, avoid Rulership (+3 stewarship) because you might get depressed and you want to keep for a while this character. Don't worry about your economy by the way, your wars will subtains your coffer, expecially with the amount of sieging you are about to do.

Your council
Check your council and if the chancellor isn't good enough for you, appoint your oldest son instead since he has a huge Diplo score and then fabricate claim on Rome ASAP, especially if you want Rome to be your capital. The other councillors are up to you, but I tend to use my marshal to train troops in the capital, Steward to collect taxes in the City of World's Desires, I keep my spymaster next to me and the court chaplain to spread culture tech. But do what suits your style except for the Chancellor.

Your kids
Educate your third son (the one being brave and ambitious) because you want him to be your heir once he is 16. The others, do what you want with them. Anthousa (your daughter), I usually marry her off matrinialary (or whatever is the word, I am French, don't ask too much complicated words for me pleas ahaha) in the Karling family because I want heir bloodlines, but you don't have too. However, if you are going Catholic, it might be more important, so consider this possibility.

What wars now?
You'll notice that you do not control most of Greece and Epirus and that Bulgaria is stupid THICC. You have two choices here : attack Attila's descendants or thoses pesky wannabe Greeks pagans. I usually choose Bulgaria, because the Duke of Karunva (the one duchy next to Constantinople) has a huge martial score and if you do not deal with him, he will raid you with like 3000 troops, causing problems for your holding(s). Holy war him and keep the duchy and everything within it except the churches (you can hand out one city if you are over your limit though). Once this boi is dead, you can turn your attention to the rest of Greece.

Another tactics is to focus Greece and Epirus and then take out Bulgaria. If you choose this route, take advantage of your council (meaning they can't vote for wars yet) and declare on all the duchies at once. They won't be in the same wars and it will mean you can't take them out one at a time. It will take longer then BUlgaria but it is worth it. Serbia might join one duchy in the war, but they are tribal and with your tech in Constantinople, they will be a pushover anyway. Do not be afraid of the pagans essentially.

We are going to ignore Naple and the Sunnis for now. If you get a claim on Rome in the meanwhile, PRESS IT!. You'll cripple Catholiscism and it is always great.

Now it's time to prepare for the civil war...

There is no real good answer here. You can let them increase Council Power for the increased vassal limits, or you can take them on and be the true emperor we all want to be. I tend to fight them, but keep the power at the same place to still have the advisor slot in the council, but you might want to give them power to keep factions at bay or you might even want to go absolute rule for the +2 holdings. Do what suits you best honestly.

ALWAYS FIGHT INDEPENDANCE FACTIONS OR CLAIMANTS OBVIOUSLY!

If you reach 1000 prestiges and the civil war is at bay, dejure claim the duchy of Benevento. Yes, the CB that uses 1000 prestige. It will make sense in not too long. You declare the war, and blitzkrieg the duchy because Lombardy might protect them since they are tributaries. Either way, you should win the war easily and you get a wopping 2 counties. What's the catch? NOw you can usurp the duchy of Benevento and once you do that, the rest of Sicily gets splitten in lonely counties and you can eat them very quickly. Bravo! You now unifed 2/5 of Italy, not that hard right? Furhtermore, you'll make a ton of money with all those wars, so that's even more perfect!

Now you character might be getting in the dangerous age, like 60. i usually try to take on the Sunnis and Antioch but you can lay low for a while and prepare for your sucession. Do not forget to vote for the right son (or daughter if you feel dangerous). With all the money you make, you can by favors and force people to vote like you for 10 years, use this!

Now the old Emperor is dead, long live the new Emperor (or Empress).
This will be the key for the game, honestly. Once you inherit, buckle up for a lot of wars.
- Convert to Orthodoxy
- Excommunicate, emprison and revoke all the cities in Constantinople and keep them. Now you have the strongest county in the World
- Continue to fabricate claim on Rome or Italy if you have the former, or Venice
- Go for Bulgaria and eat them as much as you can, but do not break the truce, we want to keep the prestige.
- Create the Kingdoms of Greece and Epirus and hand them out as vice-royalties as always
- Beat up Armenia, Antioch, Tunis, Serbia (be careful they are annoying since they are tribal)
- Build up your retenues
- Have some fun, you don't want this to be a slug

Right now, we are just trying to get prestige as fast as possible. You can create Anatolia as a Kingdom but I advise against it right now. The objective is to get to 15 000 prestige for Alexander's Bloodline. If you keep getting into wars and don't do stupid things with your characters (like pilgrim in Ireland, yes, I always end up mangle when I go there), you easily reach the 15 000 prestige required.

You have to be patient, because it is RNG based, but I never failed to get it when the event fire. Be ready to loose 10 000 prestige and 2000 gold when the event fires because you want to have the RNG Gods by your side.

If everything goes well, you will get the Bloodline and the sweet sweet invasion CB. And now you guessed it : the World is yours.

The thing with this bloodline is that you can invade only once per lifetime (once per character). You need to choose wisely in practice, but not really in fact. I usually choose to invade Lombardy because they are Christian and ti's so annoying to expand in Europe when you are orthodox.

When you invade someone, you can siege down all the top holdings of his realm and you will conquer everything and the realm goes essentially poof. This is a good way to "usurp" kingdoms aka destroy your ennemies.

Now that you have this bloodline, the game is quite easy honeslty. I'll show you what my map looks like 50 years or so in the game:

Now the game is a walk in the park. We are now too much into the game and it is too random, so I can't guide you anymore. However, you can still invade any kingdoms once per lifetime and holy war the Abbasid and to get lands that you want.

Little advices for the rest of the game, if I may :
- Don't be afraid to grant through with vice-royalties 2 kingdoms to the same character
- Don't go for absolute rule
- Have fun! You can try to RP some stuff you play tall, or anything else really!

If you have any questions, feel free to ask in the comments or if you don't like this guide, you can downrate it, it's all right. Also, please tell me if I made mistakes with my English, still trying to improve myself!

Thank you and good luck!