Stellaris

This achievement requires the Toxoids Species Pack.

You can unlock all three achievements related to the Toxoids in the same game.

When you create your empire, choose Toxoids as your race, the Relentless Industrialists as one of your civics and Knights of the Toxic God origin. This origin provides additional options for your home system. Navigate to the Name & Class page of your empire setup and at the bottom left is a field that lets you choose your home system; you can pick Toxic God's Sol to make Venus available (or you can just wait until you find a random Toxic planet).

To start the game, choose a Tiny or Small galaxy with no opponents, end-game crisis, etc.

For the achievement Could Be Worse, the Toxic world Venus will be in your home system. As you progress up the technology tree you will unlock the Climate Restoration technology. This will unlock the Detox Ascension perk, which you should choose the next time you unlock a perk slot. Terraform Venus to any habitable planet type for your empire, and then settle it. When your population is at 20 on Venus, upgrade your planet capital building to level 2 (for my empire, that was the Planetary Administration upgrade, but the name varies as you are likely aware if you are playing this DLC) and then a Coordinated Fulfillment Center (this is only available with the Relentless Industrialists civic). Open up your Policies screen and at the bottom of the list you should see Industrialism. Set this policy to Full Steam Ahead. In 30 years, the fulfillment center (which can be upgraded once to make this only take 15 years) will cause a planetary event that transforms the planet to a Tomb World. The achievement unlocks when the transformation event happens. Note that I was unable to unlock the achievement on worlds where I had Mutagenic Spas. There is nothing I can find that indicates the Spas block the achievement, but that was my personal experience. YMMV.

For the achievement Quest Complete finish the situation that is linked to your origin story. At some point you defeat an enemy that may or may not be the Toxic God, and have a choice of accepting that as the end of the quest or continuing to find the real god. You must choose continue to find the true ending as required for the achievement. 15-20 years after that decision will be the final event that unlocks the achievement.

For the achievement Toxic Workplace, after you have taken over most or all of the galaxy (including killing off the Ketlings), spin off three small (preferably one system each) sectors as vassals. When the Galactic Community prompt appears, choose to form the Community. You will have enough votes to override your vassals, so when 20 years have passed choose to create the Galactic Council, then nominate yourself as Custodian. Once that resolution passes, open the Contacts panel, choose a vassal, select Diplomacy and assign an envoy to Harm Relations; repeat for each of your vassals. This will unlock the Insult option in the Diplomacy menu. Use that once on each vassal to unlock the achievement.