As you shuffle through the Fire Emblem: Three Houses class list, you will notice a couple of special classes in the Intermediate and Advanced class recertification lists respectively. The Dark Mage and Dark Bishop classes - available only for men - require a specific item called Dark Seal. However, these Dark Seals cannot be found in shops unlike the ordinary seals, so here is the known method to obtain them.
Obtain Dark Seal from Death Knight
The only occasions these Dark Seals are available to take in the game are when the Death Knight shows up on the battlefield. You can, at the very least, encounter the Death Knight at end-of-month missions of Months 7, 9, and 11 in the first half of the story. Depending on the route you take, the number of additional times you will face the Death Knight will differ. The minimum number of times you will face the Death is 3 times on one of the game's routes, although most routes will have you take him on more than that.
As with all other lootable items, you can get the Dark Seal by either defeating the Death Knight or stealing the item from him. However, the former will prove a very difficult challenge, especially if you are playing in Hard Mode. The Death Knight is a very powerful enemy which has an Ability that allows him to counter attacks from any range. He can even kill most of your characters in one strike or two. Sword of the Creator, Luna, and skills that do extra damage to units on horseback will be your best bet if you want to attempt it.
In order to raise a male character to a Dark Bishop, you will need him to unlock Dark Mage first, so you will want to have at least 2 Dark Seals to make it happen. But although you should have an army strong enough to face the Death Knight in month 11, it is extremely risky to try beating him in the earlier missions. Read on further if you want to know how to safely take Dark Seals without risking your characters' lives.
If this page has been helpful, check out our full Fire Emblem: Three Houses guide, which has information and tips from help for beginners to advanced battle strategy.
Stealing the Dark Seal
If you want to try the stealing method, you must either have one of your characters assume the Thief class or have a character who has mastered the Thief class equip the Steal ability. This character should hopefully have a good Speed stat - at least 20 should be a safe bet in the month 7 mission - because Steal only works when the user has a higher Speed than the target. Having a Thief fast enough the first time you meet Death Knight might be a bit tricky, because you might not have anyone certified as a Thief yet, or anyone fast enough. So it might be a bit easier to get the Two Dark Seals you would need in Months 9 and 11.
Characters like Byleth, Claude, Felix, or Petra are probably most likely to be your fastest characters able to certify into a Thief. You just need Sword Skill at C and level 10. Though note Claude won't be available in the 2nd encounter with Death Knight.
After two last enemies are left in the map, have the stealer stay nearby the Death Knight - though not right beside him - and also make sure that there is an empty item slot, while all other characters should prepare to finish off the other enemy.
If you have prepared everything perfectly, it's time to work the magic. Move the stealer adjacent to the Death Knight, have him/her use the Steal command, and you can safely move the Dark Seal to the stealer's possession! Right after that, have other characters gang up on the last another remaining enemy to safely beat the mission with minimum casualties.
You can also use a character who has mastered the Monk Beginner class with the Draw Back Combat Art, to move the Thief away from harm after stealing.
If you have a character with Rally Speed (Ignatz), that can also help you out to reach the speed requirement.
Is the Dark Seal worth it?
As mentioned earlier, the Dark Seal is exclusively used by a male character to unlock the Dark Mage and Dark Bishop classes in succession, so female mage characters like Annette or Lysithea cannot become one. As the character continues to fight while assuming these classes, he will eventually attain class mastery and unlock new abilities which are as follows:
- Dark Mage class mastery bonus: Poison Strike [Ability] - If unit initiates combat and lands a hit, targeted foe loses up to 20% of max HP after combat.
- Dark Bishop class mastery bonus: Lifetaker [Ability] - Unit recovers HP equal to 50% of damage dealt after defeating a foe.
These two abilities are very useful and you can produce a very efficient male attacker with them, So while obtaining Dark Seals is a very daunting challenge, the end results will be definitely worth the struggle.