If you haven't read Part 2 yet, please go here. The purpose of this guide is to explain Act 4, starting with how to unlock it. If you're still new to the game, consider leaving this guide for a later time when you've tried out all of the characters and are regularly beating the game.

When you beat the game for the first time with Ironclad, Silent, and Defect, a small cut-scene will play showing your progress towards unlocking the keys. Once you've beaten the game with all three characters, the game will include the keys whenever you play. The three keys are ruby, emerald, and sapphire. There is no amethyst key because Watcher was released after Act 4 was created, hence beating the game as Watcher is not required.

Once the keys are unlocked, you must now collect them during your run. Keys are not saved between runs, so you will need to do this in every run in which you want to reach Act 4. Collecting keys adds difficulty to your run, so if you don't plan on reaching Act 4, don't waste time collecting them. It's also worth noting that if you collect the keys during your run and reach Act 4, the game is still considered a victory even if you die during Act 4, allowing you to progress your ascension level.

The ruby key is obtained by using the recall action as a rest site. Using the recall action uses up the rest site, meaning you can't rest or do anything else at that rest site. Because of this, it's best to save this for as late into the run as possible, as you'll have a better idea on if you're ready for Act 4 or not. The latest possible time to do this is at the guaranteed rest site before the Act 3 boss. If there is a time you reach a rest site at high enough HP and no cards worth upgrading, consider using the recall action then, as you may need the final rest site to heal before the Act 3 boss.

The emerald key is obtained by defeating a buffed Elite. Buffed Elites are indicated on the map by an Elite icon with a fire moving around it. Buffed Elites are given a buff of either strength, Metallicize, Regenerate, or Max HP. One buffed Elite will spawn per act until one is defeated, meaning if you defeat the buffed Elite during Act 1, no more buffed Elites will spawn for the rest of the run. Thus there is a benefit to defeating the buffed Elite early if possible as it will make the rest of your run easier.

The sapphire key is obtained by selecting it from any non-boss chest. Selecting the sapphire key comes at the cost of the chest's relic, indicated by the link icon that will join the two together. If you have the Matryoshka relic, only one of the two relics will be linked to the sapphire key.

There will be times where you're unsure if your run is going well enough to want to reach Act 4, in which case you can wait to collect all three keys until Act 3. Because every path has a guaranteed rest site and treasure room, the only room you'll need to target when selecting your path is the one with the buffed Elite.

Once you have all three keys collected and defeat the Act 3 boss, a small cut-scene will play showing the keys' activation and opening the door to Act 4.

Act 4's map is set in stone and never changes between runs. The path is a rest site, a shop, an Elite encounter, and the final boss. You'll most likely want to rest at the rest site and buy relics and/or potions at the shop, as your deck is most likely complete. The themes of the enemy encounters in this act are unique buffs and card order. (You'll see why shortly.)

The Elite faced in Act 4 will always be Spire Shield and Spire Spear. This is the only fight in the game where the "Surrounded" buff is applied to the player. (It's a buff to prevent it from being removable, but it's actually a negative effect.) This effect applies 50% bonus damage taken by the player from the enemy that is facing the player's back. When the player attacks or uses potions against an enemy, the player will turn to face it. Because of this unique effect, you must consider the order in which you play your cards and use your potions.

The general strategy is to defeat Spire Spear first. There may be times where to protect yourself from damage you'll attack Spire Shield and thus turn your character, but consider that this will prolong the battle and might cost you in the long run.

This encounter is also unique in that Smoke Bomb cannot be used until one of the enemies is defeated. If you have a Smoke Bomb, make sure to use it during this encounter after defeating one of the enemies as it cannot be used against the boss, and most likely your deck does not need another card compared to the possible downside of taking more damage.

The final boss of the game is the Corrupt Heart. The Corrupt Heart has two unique buffs called Beat of Death and Invincible.

Beat of Death deals you damage for every card you play. Because of this, play your block cards before your attack cards unless you have Tungsten Rod. Tungsten Rod negates the effect, which is why it's an instant buy if you're planning on reaching Act 4. Torii is another great relic to have, as the Corrupt Heart eventually buffs Beat of Death so Torii can negate this.

Invincible limits the amount of damage the Corrupt Heart can take in one turn, preventing its immediate defeat by infinite decks. Note that Vault does not reset Invincible, so you might not want to use it if you've already dealt the maximum damage for that turn.

The Corrupt Heart will start by using Debilitate, an ability that applies several debuffs and adds status cards to your deck. After that, it will follow a basic pattern where it will use either Blood Shots or Echo, then whichever of these two attacks it didn't use, then Buff. It will continue this three ability pattern until you or the Corrupt Heart die. Blood Shots is a multihit attack, so having Tungsten Rod again comes in handy. If you have Torii as well you'll be immune to this attack until the Corrupt Heart is strong enough. Echo is a large one-hit attack. Buff cleanses the Corrupt Heart of Strength Down debuffs and adds a buff depending on how many Buffs it has cast so far this combat.

Because Buff will remove Strength Down debuffs, any you cast during the combat can last at most 3 after the first turn. Any abilities that deal damage when you're hit can help a lot as Blood Shots is guaranteed to happen once during every three-turn period after the first turn. Buffer isn't nearly as useful against the Corrupt Heart due to this multi-hit attack using it up quickly, so only use it when the Corrupt Heart is using Echo.

Once the Corrupt Heart is defeated, a cut-scene will play and the game will end.

After reading this you should understand Act 4 and how to unlock it. If you have any questions or need anything clarified feel free to comment below. The next part of the guide, dealing with Ascension and achievements, can be found here.