Stellaris

You must be playing in IronMan mode and have the MegaCorp DLC enabled for this achievement to unlock.

The Galatron is a relic that can be purchased from the Caravaneers via a reliquary. In each game you can purchase up to six reliquaries using the diplomatic interaction screen. Note that you will need to have energy credits to purchase CaravanCoinz first, and then each reliquary is ~6200 CaravanCoinz. Per the official Stellaris wiki, there is a 1 in 200 chance (0.5%) of any given reliquary containing the Galatron, and you can purchase one every 500 days.

In the event you don't get this while aiming for other achievements, you can grind both Galatron-related achievements (and the What Was Will Be achievement if you need that as well) by setting up a Tiny galaxy (200 stars) with no AI, no FE, no Marauders, Hyperlanes set to max, Primitives to a value of 2 or more and Caravaneers to On.

When you run across the Caravaneers, you will need to accumulate around 20k energy to purchase their coins and get the six attempts at unlocking the Galatron. If you get the Galatron, raise up a primitive civilization and release it from vassalization. After the truce period, save your game and return to the menu. Load up your game in multiplayer, choose your ex-vassal as the empire to play, and declare war on your normal empire with a war goal of "Take Galatron." Save your game, load it up in multiplayer again, and this time choose your primary empire. Surrender the Galatron immediately, and then save and go back to single player. At this point, wait out the ten year enforced truce period and then attack the former vassal with the same war goal and either destroy them or wait for them to surrender and get the Galatron back.

If you are NOT also grinding for What Was Will Be, you can make a copy of your save file prior to opening your 6th reliquary and keep reloading it. This is much faster than starting a new game every 15-20 minutes, but does not work to grind the What Was Will Be achievement.