Fusion Hammer Slay the Spire

Fusion Hammer in Slay the Spire is one of the more controversial boss relics due to its powerful yet restrictive nature. As a relic that can dramatically alter the way a player approaches deck-building, Fusion Hammer grants a substantial benefit but comes with a serious drawback. In a game where strategic decisions and adaptability are key to survival, understanding the true value of Fusion Hammer can make or break your run. Whether you’re playing Ironclad, Silent, Defect, or Watcher, the choice to take this relic should be based on your deck’s needs and your willingness to adjust tactics.

What Is Fusion Hammer?

Fusion Hammer is a boss-tier relic that provides a major energy boost but limits card upgrading. Specifically, when acquired, it grants +1 Energy at the start of each turn but disables all access to Campfire upgrades. This means you can no longer use Rest Sites to enhance your cards, removing a critical progression option.

The energy boost makes Fusion Hammer appealing, especially in energy-hungry decks that rely on playing multiple cards each turn. However, the inability to upgrade cards forces players to seek alternative ways to improve their deck’s power, such as finding upgraded cards through events or other relics.

Relic Description:

• Name: Fusion Hammer
• Effect: Gain an additional energy at the start of each turn. You can no longer Smith at rest sites.
• Type: Boss Relic

Strategic Value of Fusion Hammer

In Slay the Spire, energy is a critical resource that limits your actions each turn. Gaining a permanent extra energy significantly increases your options, allowing you to play more attacks, blocks, or powers. This is especially effective in:

• Decks with high card draw– More energy means more utility from the extra cards.
• Power-focused decks– Once powers are in play, the energy can be devoted to offense or defense.
• Zero- and low-cost cards– Having extra energy allows more flexibility with costly cards that might otherwise be dead in hand.

However, Fusion Hammer’s restriction makes it unsuitable for certain builds, particularly those that rely on upgrading key cards like Inflame, Cloak and Dagger, Dualcast, or Establishment. Decks that depend heavily on upgraded cards to scale effectively will suffer without the ability to Smith them.

Fusion Hammer Synergies by Character

Ironclad

Ironclad decks benefit from energy to sustain aggressive playstyles. Fusion Hammer can be a strong pick if your deck already has upgraded cards or if you’re leaning into a strength-build with many commons that don’t necessarily need upgrading.

Best synergies include:

• Body Slam– Doesn’t require upgrades to be effective if paired with high block sources.
• Power-based decks– With cards like Juggernaut or Demon Form, upgrades are less crucial.
• Exhaust decks– The ability to play more cards each turn strengthens the cycle and utility of exhaust synergies.

The Silent

For the Silent, Fusion Hammer can work well with poison or shiv decks, depending on the structure. Shiv decks in particular love extra energy since the payoff scales with the number of attacks per turn. However, many key Silent cards benefit significantly from upgrades.

Best synergies include:

• Poison builds– If you already have Catalyst+, you won’t need Smithing much more.
• Shiv synergy– With relics like Kunai or Shuriken, more attacks per turn mean stronger scaling.
• Discard synergy– Extra energy helps you cycle and react more efficiently.

The Defect

The Defect is perhaps the most energy-hungry character, making Fusion Hammer especially attractive. Orbs, especially Lightning and Frost, often require energy to evoke and loop effectively. If your deck is focused on channeling and powers, this relic can make it unstoppable assuming you don’t rely heavily on upgrading.

Best synergies include:

• Power builds– Electrodynamics and Biased Cognition don’t require upgrades to dominate.
• Frost/Block decks– More energy = more orbs = more block or attack triggers.
• Echo Form synergy– High energy lets you duplicate high-cost cards more easily.

The Watcher

Fusion Hammer can be a risky pick for Watcher due to her reliance on key upgrades for stance synergy. Many of her most powerful cards, such as Blasphemy and Lesson Learned, gain immense value from upgrades. However, with the right build, especially if you’ve already upgraded core cards, Fusion Hammer can fuel devastating combo turns.

Best synergies include:

• Draw engines with Rushdown– The extra energy powers large chains of actions.
• Mantra builds– Less reliant on upgrades and benefit greatly from energy.
• Establishment decks– With enough setup, you’ll need energy to make the most of returned cards.

When to Avoid Fusion Hammer

Despite its benefits, Fusion Hammer can be a trap in many situations. You should avoid it if:

• You are early in the run and still have many cards needing upgrades.
• Your deck relies on upgrade-based scaling (like Catalyst, Armaments, or Fasting).
• You lack other ways to get upgraded cards (like Toxic Egg or Events that upgrade).

Moreover, if your deck is already streamlined and relies on high-efficiency upgraded cards, then Fusion Hammer can become a liability by removing your ability to fine-tune your deck further.

Counteracting the Smithing Loss

If you do decide to take Fusion Hammer, there are several strategies to mitigate the loss of Smithing:

• Toxic Egg: Automatically upgrades skill cards when added to your deck.
• Molten Egg: Does the same for attack cards.
• Medical Kit or Gearia events: Some events can upgrade cards outside of campfires.
• Pray at events: A few random events can offer upgrades or remove unupgraded cards.

Utilizing these opportunities becomes essential to maintain deck strength throughout the Act 2 and Act 3 encounters.

Fusion Hammer in Slay the Spire is a high-risk, high-reward relic that can power up the right deck while crippling others. It shines in situations where energy is the limiting factor and upgrades are already handled. However, it can stall progress for decks still dependent on card enhancement. Knowing when to take Fusion Hammer and when to pass is a skill that separates experienced players from newcomers. Always assess your deck’s synergy, progress, and upgrade potential before choosing it, and with the right build, Fusion Hammer can turn a good deck into a game-winning powerhouse.