Stellaris

This achievement requires the Leviathans DLC to be enabled, and as with all achievements, Iron Man mode must be enabled. It also is somewhat glitchy as it does not unlock every time even after fulfilling the known requirements.

It does NOT require the Federations DLC despite the requirement to be in a federation, although the DLC takes some time pressure off at the end if you have it enabled.

I strongly recommend reading the official Stellaris Wiki entry on the War In Heaven, which is what this achievement refers to: https://stellaris.paradoxwikis.com/Fallen_empire#War_in_Heav...

To unlock Last, Best Hope you need two Fallen Empires to awaken and fight each other while you remain neutral and lead a federation which then destroys at least one of the two Awakened Empires. The key parts are:
1) you must have two Awakened Empires that declare a War in Heaven
2) you must stay neutral when offered the choice of joining either side or fighting both.
3) you must then join (or already be a member of) a Federation when the option is presented. This Federation will be renamed to "League of Non-Aligned Powers".
4) You must wait for the two Awakened Empires to declare war on your Federation (if you declare war first, it voids the achievement).
5) you must be the President of said Federation when you destroy the Awakened Empires. Despite the description of the achievement, you must destroy both and all of their vassals (if any) before the achievement will unlock.


This achievement is not possible to unlock in every game, but you can maximize your chances by setting up the starting galaxy as follows:
Starting Size: Huge [1000 stars] -the minimum size that permits 4 FEs
Fallen Empires: 4
Starting AI: 0
Primitives: Off
Marauders: Off
Crisis Strength: 0.25
Crisis: Prethoryn Scourge

Mid-game, End-game, Victory: 2400-2450-2500. Feel free to adjust upward if you want to conquer more of the galaxy, but adjusting downward is likely not going to give you enough time to set up for the achievement.

Not a requirement, but strategically this will be easier if you set wormholes to 0, Gateways to max, and hyperlane density to 2.5.

Starting Empire: Not using the origin story Scion, and preferably not Pacifist or Fanatic Pacifist (because you have to declare war eventually unless you want to rely on the Awakened Empires declaring war on you). Choosing Authoritarian or Fanatic Authoritarian gives you more control in the end-game by permitting Hegemony as a Federation choice if you have that DLC owned and enabled. This guide doesn't work if you use a gestalt empire (machine or hive-mind) so bear that in mind. You can still do it, and most of the advice still applies, but you can't siphon off sectors to create vassals with a gestalt empire, so you need to have other sources (AI or Primitives) to be able to form a federation in the end-game.

The Stellaris wiki entry is a little unclear on the math, but at best, two Awakened Empires have a maximum 45% chance of both changing from FE to AE and then declaring the War in Heaven. It is possible to interpret the wiki to mean that IF AND ONLY IF two FEs change to AE that then you have a 45% chance of the War in Heaven happening. The chances of two FEs awakening appears to not be very high, but having an end game crisis improves the odds and the Prethoryn Scourge is one that is easy to keep isolated but still alive. This is important because an FE won't awake unless the Crisis is undefeated for at least five years. Use the galaxy geography strategically and don't kill them off, just wait for an FE to awaken and come attack them.

For the War in Heaven to happen, two Fallen Empires need to transition to Awakened Empires and not be at war. You can trigger them awake such as by destroying a holy world, but the War in Heaven won't trigger unless/until two AEs are not in an active conflict with anyone in the galaxy. It is possible for two FEs to awaken and have a rivalry that leads to war, but NOT trigger the War in Heaven. You will get a specific series of pop-ups if the War in Heaven is triggered, including one that states a FE awakens because of an old rivalry and the specific declaration about the War in Heaven.

Play the game focusing on expansion and population growth over all else. Also, leave a buffer zone around all the FEs and focus on capturing the rest of the galaxy. This will ensure that any AEs expand their territory into areas adjacent to other FEs and AEs rather than into areas you are trying to control. Generally, plan ahead so that you control choke points and all the territory you need for your economy rather than having groups of 2-3 systems controlled by AEs right in the middle of your empire. You will eventually need at least one partner to create a Federation with, but you can create that when the need arises.

Regarding the end-game, ensure that your fleet power is greater than 120,000 to reduce the time for a Fallen Empire to awaken. Building a Colossus is a cheap way of starting a war since you don't have to spend any influence, and your tech level and military power should far exceed the Awakened Empire strength.

You do NOT need to research Federations for this to work. As long as you wipe out all other non-FE empires as you expand - such as the Ketlings - you will be offered the chance to create a Federation shortly after the War in Heaven starts. You DO need a second empire to join the Federation for this to work, so as soon as a Fallen Empire awakens, pick a weak sector as far from the FEs and AEs as you can, and on the Sector menu choose the Create Vassal option. Assign a few envoys to improve your relations and let it go independent. If the War in Heaven happens, they will join your Federation and if you picked Authoritarian as an ethic you should be able to create a Hegemony for the Federation with you as the President. If you didn't pick Authoritarian you will be part of a Galactic Union Federation and while you should still be President (which you need for the achievement) you will only have 20 years before that cycles.

When the War in Heaven starts, you will be offered the chance to join either side or stay neutral. You MUST choose to stay neutral. Shortly after declaring your neutrality, the Federation will be formed and named "The League of Non-Aligned Powers". You must wait for the AEs to declare war on you (there will be a special pop up specifically for this) and then complete a Total Annihilation victory against both empires and any vassals they may have. The achievement should unlock after the last system is taken over.

Last note: To reduce the chances of civil war at an inopportune moment, I recommend setting Robots to Forbidden in the Policy menu, or at the least don't research Synthetics. If you have Robots as forbidden, you can still use the automatic research function and Synthetics will never show up.