Black Alice is a returning character from the original trilogy and is one of the three Monster Lords battling for the title after the disappearance of the 16th.
She is first mentioned by the chief elf in Enrika.
The chief elf brings up Black Alice. She is the second of the three Monster Lords battling for the title, after to the sudden disappearance of Alice the 16th. With Black Alice being the most warlike of all of the Monster Lords she naturally attracted a unique group of monsters. The monsters who opposed peace and co-existence. She has a smaller army overall, compared to the 15th's, but arguably the more powerful individually.
The elf mentions that while Black Alice hasn't entered conflict with Alice the 15th's faction yet, it's only a matter of time before they do.
Black Alice is briefly seen in the Puppeteers Tower.
After the party leaves the tower, Kagetsumugi returns to the throne room.
A figure asks what Kagetsmugi learned from her short encounter with Luka, but there was not enough time for Kagetsmugi to make a judgement on his power, thanks to Ilias/Alice abruptly dragging him out of the tower, much to the disappointment of the figure. Kagetsmugi declares that she is unable to judge what the figure is really like either, who is revealed to be Black Alice.
Black Alice addresses Kagetsmugi as the last puppeteer, the fourth Kagetsmugi. It is not a name, but a title honoured to the leader of the clan.
Kagetsmugi is in debt to Black Alice as she was the one who created the Kagetsmugi division, over 500 years ago. She is naturally loyal to Black Alice and provides her services to her exclusively.
Black Alice brings up how her dream to control the world was ruined twice now. The first time by Marcellus, the second by Luka. She finds it amusing how regardless of her consecutive failures, she continues to pursue her dream. Even after being infused with the power of a god via the White Rabbit drug, she was still unable to defeat Luka. Kagetsmugi is amazed that such a feeble looking boy would have the power to beat Black Alice in any timeline.
Black Alice prepares to summon a variety of some of the most powerful monsters. The ones who reigned throughout the Great Monster Wars. Azi Dahaka, Sun Wukong, Quetzalcoatl, Tezcatlipoca, Izanami, and Himiko. All named after deities of various beliefs.
Kagetsmugi is impressed, and she unveils her creations. Alipheese the 9th - Black Alice takes particular pleasure in seeing her younger sister in this sorry state - Alipheese the 11th+12th - they were missing too many body parts so she was forced to join them both into one being, but Black Alice likes them better this way - and Alice the 6th. Kagetsmugi has more Monster Lords being made for Black Alice, but she will have to wait longer for them. Despite gathering their bodies, they are heavily damaged.
Black Alice is particularly pleased with Kagetsmugi, and believes that with their forces combined, they will surely start an "enjoyable tea party."
- If you hadn't already picked up on it, this is the same Black Alice from the original trilogy. Like Ilias, she took the White Rabbit drug, and they both turned up in the Paradox universe. Now, there was someone else who took that drug too. If that person is who I think it is, Black Alice is likely working with her, as they got on very well in the main universe.
- Since this Black Alice is from the original trilogy, it means that this world's Black Alice is still around. So we might be seeing two Black Alice's. Assuming that Ilias resurrected her, and Black Alice survived the destruction of Heaven due to the Tartarus.
- Finally, there was an image of the Armored Berserker along with Black Alice in the paradox trailer (see 3:21-3:23). Not fighting, but standing back to back. It is possible that they're allied with each other. It also means there's the possibility of something deeper going on between them.