If you've sunk hundreds of hours into the roguelike card-battler Slay the Spire, then you've probably defeated the Act Three Boss (Awakened One, Time Eater, Donu and Deca) or gotten fairly close. But even if you have, don't think the game is over just yet, as there is still one more secret to Slay the Spire you have yet to unlock.
After beating Act Three for the first time, a new set of items will be available for you to obtain. These are the Ruby, Emerald and Sapphire Keys. Each requires you to complete a different task to obtain them, but if you can gather them all before reaching the Act Three Boss in a run, you will unlock an addition act where you can fight the Act Four Elite, Spire Shield and Spire Spear, as well as the true final boss, The Corrupt Heart. However, getting each key is no walk in the park.
The Ruby Key is found at the rest site. Typically, the rest site lets you either rest to gain back health or smith to improve cards. However, you can decide to forgo both options and obtain the Ruby Key instead. Be considerate about when you should do this, as regaining health is a hard thing to do in Slay the Spire. Getting it sooner rather than later might be in your best interest.
The Sapphire Key can be found in treasure rooms. In each Act, you'll encounter at least one treasure room, which offers you a relic. Relics are items that give passive abilities, such as an increase to your strength or defense, or giving you extra energy whenever you shuffle your deck. However, with the keys in place, each treasure room will now offer you the choice between a new relic or taking the Sapphire Key. Occasionally, you'll encounter a mystery room that could randomly generate a treasure room, enemy or random encounter, but don't bank on them. There may be times when you're offered a relic you'd like to choose over the Sapphire Key. While it may be fine to pick the relic sometimes, be sure you don't waster all of your chances at getting all three keys.
Finally, there's the Emerald Key, which may be the most difficult to obtain. Each Act in a run will now have a single Elite enemy who's stronger than average. You can tell which one the Elite is by the fire animation around its icon. These enemies have different buffs, such as extra strength or health regeneration. Defeating this Elite will earn you the Emerald Key. Go after them sooner rather than later. The enemies get stronger with each Act, so you'll be in for a tougher fight if you put it off to the end.
Once you have obtained all three keys and defeated Act Three's boss, you'll open the gate to Act Four. As you may expect, this secret final act offers a challenge that is near insurmountable. You'll want to regain all your health, revisit a rest site and go to a shop before fighting the last two bosses. First up are the Spire Shield and Spire Spear, which you'll fight simultaneously. The Shield builds up defenses for itself and the spear, and it can also bash attack for major damage. The Spear chips away at your health with multiple attacks per turn that add up quickly.
Finally, there's the strongest boss in the entire game: Corrupt Heart. The Heart can make you take damage for every card you play and even has a cap on how much damage it can take in a single turn. It can even generate Wounds and debuffs and when it attacks, and its attacks can either deal a lot of damage at once or chip away with over 10 attacks. Be ready for the fight of your life.