Mercure’s Lab - How to Unlock & Use

Mercure’s Lab Guide for Seven Knights Re:Birth. See how to unlock Mercure’s Lab, Dark Predictions, Black Magic Experimentations, Hero Forecasts, Slot Rates, and Leah in 7K ReBirth.

How To Unlock Mercure’s Lab

Clear Adventure Nightmare 5-10

Mercure’s Lab unlocks after clearing Nightmare Chapter 5 Stage 10 in Adventure mode. Unlocking the Lab serves as a reward for progressing through the more challenging Nightmare content.

Can Be Accessed in the Menu

Mercure’s Lab can be accessed through the Main Menu → Mercure's Lab once it has been unlocked by clearing Adventure NM 5-10. This tab becomes available in the Workshop menu after completing the required stage and allows you to begin hero experiments.

Mercure’s Lab Uses

Unlock New Heroes

Mercure’s Lab is especially useful for players aiming to complete their Hero Codex. By sacrificing unused or duplicate 6★ heroes, you can obtain new heroes that you may not have unlocked through regular summoning.

Fill Up Hero Codex

Since the results from the Dark Predictions often include a wide range of heroes, including those that are harder to obtain. It increases your chances of discovering missing entries in your Codex, helping you earn Rubies.

Expand Your Roster

Mercure’s Lab helps you grow and improve your hero collection by letting you trade in weaker or duplicate heroes for new, stronger ones. This way, you can keep upgrading your team by getting rid of heroes you don’t need and making space for better heroes. It’s a great tool to keep your roster strong and ready for tougher challenges as you play.

How Mercure’s Lab Works

Steps for Mercure’s Lab

Material Selection

When selecting materials, players can choose up to five 6-star heroes and Leah to use as materials. The choice of hero materials is important, as each hero used contributes to your overall score based on their value and type. Using higher-quality heroes will result in more points, helping you achieve a higher rank.

Points Earned by Material Hero

Some Heroes Cannot Be Selected

Not all heroes are eligible for use in Material Selection. Specifically, heroes categorized under the Seven Knights of Old cannot be added as material.

Substitute Leah as Material

Leah can be used as a material in place of a standard hero, providing greater flexibility when forming your material combinations. Including Leah in your material lineup gives you more room to strategize, optimize your score, and explore various possibilities without compromising your main roster.

No Need to Rank Up 4-5 Star Legendary Heroes

Gold is required to register 4-5 Star Legendary Heroes. The amount of Gold needed is less than what's required for Rank Up, and you won't consume any Rank Up materials, so there's no need to Rank Up 4-5 Star Legendary Heroes.

Auto Add Settings Is Convenient

You can configure Bulk Registration settings from the icon to the right of the Bulk Registration button. If you set the rarity for all 5 registration slots in advance, you can register materials just by pressing the Bulk Registration button, which is extremely convenient, so be sure to make use of it.

Quality‑of‑Life Improvements

After the recent update, a filter function has been added to the material selection screen. It allows you to exclude heroes who have completed Transcendence, are equipped with gear, or have been powered up. This helps prevent accidental selection of valuable or prepared heroes.

Dark Predictions

Dark Predictions let you see which hero you might get. Only one hero from the list can be chosen. You can use Predict Again to get a new list, but this will erase the current results. Running a Dark Prediction costs 300,000 Gold per attempt.

Hero Forecast

Each prediction costs Gold, and the chances of getting better heroes depend on your rank. The strongest heroes appear in the last slots, so keep predicting if you're aiming for the best.

Slot Rates

The heroes are placed into slots based on their rarity. Higher rarity heroes go into slots with lower chances. Heroes are arranged by rarity from right to left, with the highest-rarity heroes placed on the right, and the rightmost slot has the lowest appearance rate. Only one Seven Knight of Old can appear at a time. If a Seven Knight of Old is picked, the other heroes are chosen with updated chances.

Tips!	The appearance rate of the 5 slots differs depending on rank. This image shows Rank 7.

Check Rate Info

For more details on how heroes are chosen, tap the Rate Info button. It shows the chances for each hero and slot. Keep in mind that the numbers are rounded, so they may not add up to exactly 100%.

Setting Up Auto Dark Prediction Is Convenient

If you set your desired hero using the Auto Dark Prediction button, it will automatically repeat Dark Prediction until that hero appears. Keep in mind that this consumes a large amount of Gold, but it's a convenient feature.

Increase Odds for the Rightmost Slot

If you want to increase the odds for the rightmost slot, press the "%UP" button. Black Magic Experimentation will then require an additional 1,000 Rubies, but the odds for the other 4 slots will decrease while the odds for the rightmost slot increase.

Auto Predict Improvements

In the 12/23/2025 update, the Auto Predict Button will now be displayed immediately when performing an Auto Prediction.

Black Magic Experimentation Results

In Black Magic Experimentation, you get one hero from the Dark Predictions. However, take note that any heroes you use as materials will be gone for good. Also, any gear on those heroes will disappear. When you get your hero, all the prediction results will be erased so it's important to choose carefully.

Best Materials for Mercure’s Lab

Aim for Rank 6-7 or 17-18

Best Material Combinations by Rank

From Rank 6 onward, Rare Heroes stop appearing, and from Rank 17 onward, both regular Legendary Heroes and Rare Heroes stop appearing. It's best to combine materials so you exceed one of these thresholds.

Detailed Dark Prediction Rates by Rank

See More

Get Material Heroes Through Fusion or Rank Up

Material Heroes must be 6-Star, so use 5-Star Fusion or Rank Up 4-Star Heroes to obtain 6-Star Heroes. Even after you've Ranked Up every hero you use regularly, you'll still need the Power Up Dungeon, so be careful not to let your Stamina overflow.

Examples of Material Heroes

• Heroes no longer needed after reaching Transcendence 12
• Heroes you rarely use
• Heroes that are fine without any Transcendence
• Heroes already at Transcendence 6 that you only use outside PvP or Guild Content

Ideally, material heroes should be ones you no longer need after reaching Transcendence 12, but it's also a good option to use heroes that are fine without Transcendence (such as support specialists or heroes who only shine in the Power Up Dungeon), or heroes already at Transcendence 6 that you don't use in PvP or Guild Content.

What is Mercure’s Lab?

High Risk and Reward System

Mercure’s Lab is a special feature in Seven Knights Rebirth that allows you to sacrifice existing heroes to obtain a new, possibly better hero through a dark experiment. All used heroes and their equipment will be lost permanently.

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