The Human Haven keycard is a secret item you can earn in Squanch Games' High on Life. After you've defeated Garmantuous, you'll immediately be met with the game's scrolling credits. Once these have finished, you will be met with the game's starting screen. If you choose to continue, you'll start in your house with Gene commending you for having defeated such a rigorous boss. Besides the other end-game things you can do, such as find different Gatlian mods, you can search for High on Life's secret ending. The only thing you'll need to do is acquire the Human Haven keycard.
How to get the secret ending in High on Life
After continuing High on Life, Gene will mention that the rest of the universe doesn't know what's happened yet because news tends to travel slowly in the galaxy. In fact, no one around you seems to really care that you've defeated bosses like Douglas or Krubis. You will immediately want to travel across different worlds using the Bounty-5000 in your house. Your specific point of interest will be Clugg's Office within Nova Sanctus. Here, you will find a keycard on Clugg's desk that you'll want to swipe called the Human Haven keycard.
After this, you'll want to head back home and recalibrate the Bounty-5000 to instead head over to Human Haven within the Unknown Sector. Inside, you'll want to use your jetpack to fly up toward a door with a red light. The light will be bright enough to see from the ground floor. Once you reach this door, you'll be able to open it up to the gruesome sight of dead humans. You'll soon find out that Clugg was never on your side and was, in fact, working with another, much more powerful alien.
We don't want to spoil the surprise for you, so you'll just need to check this out yourself! Once you've met with the aforementioned alien, you'll be brought back to the fight floor of the Human Haven. At that point, you'll earn the achievement titled Sequel Bait.
For more High on Life, check out Should you make Gene leave or stay in High on Life? only on Pro Game Guides.