Stellaris

This is a straightforward but lengthy achievement. There needs to be a non-player empire as Galactic Emperor, which you then overthrow in a rebellion as leader of a federation. You do not need the Federations DLC nor do you need to unlock federations via Traditions.

Start a game on a small map (400 stars) with no AI empires, no Fallen Empires, no Marauders, but Primitive Civilizations set to max. Take over the entire galaxy, setting up observation posts on all planets with non-xenophobe Primitives, and if the Keflings spawn wipe them out to avoid trouble later on. Some primitives may move to Space Age along the way, which is fine, but generally your goal is to have your galaxy-spanning empire and a lot of single-star empires. Along the way, ensure that you choose as a tradition the Subterfuge option as that makes Espionage actions faster and easier.

Once you are prepped, use your Observation Posts to advance all of the Primitives to be your Vassals. As soon as they become a Vassal, use the Diplomacy menu to release them and then when you can (in 10 years) close your borders with each. This should minimize the chances of them getting into wars with other ex-Vassal empires and wiping out your networks. Speaking of networks, you need to create Espionage networks on as many primitives as you can, and try to get at least one Asset with the blue Subterfuge skill in each network. Depending upon your empire choices and research, there are buildings and edicts that can get you more envoys. Build a Gateway in an adjacent system for each empire so that you can quickly move fleets to attack them in the last phase.

Shortly after the second primitive is uplifted, you will have the opportunity to form the Galactic Community - this is a perquisite for Sic Semper Tyrannis, so say yes. If it fails, it will pop up again in 20 years and you should easily get 3 yes votes at that point. 20 years after the Galactic Community is created, you can vote to create the Galactic Council, which is the next step. During this 40 year span, pay attention to the diplomatic strength of each empire and ensure you are building Espionage networks on each of the top four. The rest can be disbanded at this point. You only need one, but you won't know which one yet and they take a while to level up. Ideally, you want your infiltration level at 100 on each of the four empires with the most diplomatic weight. You will also want to have enough full-strength military fleets and transport fleets to simultaneously attack 2/3 of the empires in the galaxy.

20 years after the Galactic Council is created, empires can nominate themselves to be the Galactic Custodian. You are not able to nominate other empires, so pick one that is on the Galactic Council where you have a strong Espionage network, and remember (or record) the empire name. Save your game and return to the menu. Choose Multiplayer, Host Saved Game, set a password so that nobody joins you and messes with things, and then choose the empire that you recorded earlier. You don't need to unpause the game, just go into the Galactic Community menu and nominate that empire to be the Galactic Custodian. If they have a pending proposal already, you will need to withdraw it. Save and return to menu, choose Multiplayer, Host Saved Game, set a password, and then choose your initial empire, using that to support the motion to nominate the other empire as Custodian. At this point you can then save and quit, and return to single player. Your diplomatic weight will be so high compared to the other empires that which ever way you vote on anything will always win, so the target empire will definitely be the Custodian once the Senate session reaches its end date. You can disband all other espionage networks at this point as you will need your envoys momentarily. A subterfuge-skilled Asset in your network against the Custodian will definitely speed up the revolution later. Your infiltration level against the Custodian needs to be close to 100, so keep an eye on it and recruit additional Assets to boost your upper limit if needed.

Swap empires again in multiplayer, and this time submit the petition to remove term limits from the custodian, and then back to your empire to support it. Once that passes, swap one more time to submit the proposal to Proclaim the Galactic Imperium. When this passes, the Galactic Community ceases to exist and is replaced by the Galactic Imperium, with the prior Custodian now the Galactic Emperor.

With the Galactic Empire formed, it is time for revolution! The first step is to weaken the Imperial Authority of the Galactic Empire. It starts at 100 and will increase due to envoys assigned to building it up. You will want to assign all of your envoys except the one running the espionage network against the Emperor to weaken the empire. You do that on the Galactic Empire interface screen (looks very much like the Galactic Community interface) on the first tab, upper right corner, there is a raised fist icon with a red down-arrow. Click on that and assign all of your unused envoys. Open the Espionage screen and you will need to run the operation to reduce the Imperial Authority. It will be second from the bottom, labeled Imperium: Weaken Imperial Authority. After 3-4 repetitions of that operation, you can choose the last operation labeled Imperium: Spark Rebellion. Use an Asset with subterfuge to speed this up as it has quite a few steps. When that operation succeeds, the empire will split into two factions. The Emperor and a few empires that support it, and then you with the rest of the empires formed into a Federation with you as the leader. You will already be at war, so wipe out their fleets and capture their planets. You will need to wipe out enough of the Emperor's affiliated systems to force a surrender (this may mean total annihilation). Once you win the war and clear out all the messages that pop up as a result, the achievement will unlock.