All existing Dark Spells take up one housing space in the spell storage, whereas most Elixir spells take two spaces. The total spell storage capacity is shared between the Spell Factory and the Dark Spell Factory. The Dark Spell Factory adds one spell storage capacity to the total spell storage capacity upon construction, but adds no more when it's upgraded.
After the December 2015 update, players can donate one Dark Spell to clanmates if their Clan Castle's level is 4-6, in which it has one Spell housing space. Two can be donated if the Clan Castle is level 7-8, in which it can hold 2 Dark Spells as it has 2 Spell housing space.
The Dark Spell Factory is unlocked at Town Hall 8.
The Dark Spell Factory undergoes a significant visual change at all levels.
When initially constructed, the Dark Spell Factory consists of a big wooden cauldron with a red interior and metal rims, supported by 4 wooden legs which have metal bases. Unlike the Spell Factory, it doesn't have a slingshot-shaped structure for the mixing spoon.
At level 2, the base is now metal with golden spikes on the legs.
At level 3, the outer side of the cauldron receives gold plating. The legs seem thicker and is protected by an external metal plating outside, which has crossed bones stuck on it.
At level 4, the metal plating covering the legs becomes larger, and three large golden spikes replace the crossed bones. The top of the cauldron also becomes covered in gold, and the four points where the legs attach are covered in a light blue.
At level 5, the golden spikes are removed from the base, the light blue rim coloring is replaced with gray with a diamond on top, and the bases are sharpened on the top.
The Dark Spell Factory's styling cues are inspired by the Dark Barracks.
The Dark Spells brewed in the Dark Spell Factory are placed inside vials, unlike Elixir Spells, which are placed inside bottles.
Since there is no spell counterpart of the Army Camp which can hold spells, the Spell Factories takes the role of both creating and holding spells; the Spell Factory determines the main part of spell capacity, while the Dark Spell Factory increases the capacity by one. For example, you'll have to upgrade the Spell Factory to level 5 (rather than upgrading the Dark Spell Factory) in order to hold eleven Dark Elixir spells.
However, as of the 12/10/16 update, if you do this, you can no longer simultaneously hide Elixir by queuing Elixir spells.
Although Dark Elixir Troops generally have more housing spaces than ElixirTroops, currently all Dark Elixir Spells occupy only 1 housing space, while most Elixir spells have at least two. This probably leaves room for future Dark Elixir spells which are more powerful and take more than two housing spaces.
Oddly, although the Skeleton Spell is unlocked at Dark Spell Factory level 4, the crossbones appear on Dark Spell Factory level 3. Also, the level 4 Dark Spell Factory, which is unlocked at Town Hall level 9, has similar spiked bottom as the level 14 Cannon and Archer Tower, the level 10 Mortar, the level 5 Bomb Tower, and the level 2 Eagle Artillery, which are unlocked at Town Hall level 11.
If the player only brews Elixir Spells but not Dark Spells, the Dark Spell Factory will still play its brewing animation. The inverse is also applicable.
Tapping this icon displays information about the Dark Spell Factory, such as Level, Spell Capacity and Hitpoints.
Tapping this icon begins upgrading the Dark Spell Factory to the next level, if you have enough resources and a free Builder. When the Dark Spell Factory is at maximum level, this icon is not shown.
Tapping this icon instantly upgrades the Dark Spell Factory to the next level, at the cost of one Hammer of Building if you have one. When the Dark Spell Factory is at maximum level, this icon is not shown.
Tapping this icon, which is displayed only while an upgrade is in progress, instantly finishes that upgrade at the cost of the displayed number of Gems.
Tapping this icon, which is displayed only while an upgrade is in progress, instantly finishes that upgrade at the cost of one Book of Building. This icon is only shown if you have at least one Book of Building.
Tapping this icon, which is displayed only while an upgrade is in progress, boosts all your Builders for 1 hour by a factor of ten, at the cost of one Builder Potion. This icon is only shown if you have at least one Builder Potion.
Tapping this icon, which is displayed only while an upgrade is in progress, cancels the current upgrade and refunds half of the upgrade cost.
Tapping this icon boosts your spell creation for 1 hour, reducing the time required to create spells by a factor of four for the duration of the boost.
Tapping this icon boosts your troop training, spell brewing and hero regeneration for 1 hour, reducing the time by a factor of four for the duration of the boost, at the cost of one Training Potion. This icon is only shown if you have at least one Training Potion.
Tapping this icon opens up the Dark Spell Factory's interface, allowing you to brew spells.