|This article is about The realm where Vampires primarily dwell. You may be looking for The sixth chapter of The Case Studies of Vanitas.|
Altus is an alternative dimension from the world in Jun Mochizuki's The Case Study of Vanitas, in which Vampires prominently dwell. Altus appears to be a dark reflection of the human realm, bearing significant similarities overall with minor differences; such as the sky appearing to be "cracked" and a large castle belonging to the ruling Vampire aristocracy looking over Altus Paris.
In order to reach Altus, one must use specialized passages known as Borders, which were created as the result of The Babel Incident. However, only Vampires created by The Babel Incident as well may access them, thus any other individual wishing to enter Altus must be holding on to a Vampire created by The Babel Incident, or risk being lost between dimensions.
After Dominique de Sade and Chrysler arrive at Galerie Vivienne, Dominique shackles Noé Archiviste in a collar and leash and guides him to a bookstore, where they access one of the Borders hidden there in order to enter Altus. Dominique's motivations for doing so include her concern for Noé in relation to his involvement with Vanitas and his ongoing investigation into Curse-Bearers, while also having been invited to a Bal Masque in which Lord Ruthven is attending, thus offering Noé and opportunity to seek information about Curse-Bearers from him.
Before crossing over however, Vanitas - who had followed Dominique, Chrysler and Noé, calls out to Noé as he jumps through the Border. Knowing what could happen to Vanitas if he were left behind in the Border, Noé quickly grabs Vanitas mere moments before being transported to Altus. After Noé scolds Vanitas for the risks he'd taken, Dominique leads Chrysler, Vanitas and Noé outside into Altus Paris - asking Vanitas why he followed her and Noé in the process; only for Vanitas to grin and reveal that he'd done so because Dominique had attempted to separate him from Noé. Outside - Noé is starstruck by Altus Paris - though he cannot wander too far because of Dominique's leash. With Noé distracted, Vanitas asks Dominique about her grandfather - revealing that he doesn't know much about The Master, apart from the fact that he's known as the "Shapeless One" and is feared by many. Vanitas further states that he'd underestimated Noé, not realizing that he had ties to the aristocracy, and asking Dominique if Noé's appearance is also deceiving and whether or not his parents were truly the rulers of the otherworld. Dominique stops Vanitas abruptly, while not facing Vanitas, Dominique reveals to him that Noé is actually an orphan who had been put up for auction on the black market as an Archiviste survivor, and that her grandfather had purchased him.
This shocks Vanitas - who remains silent as the three of them make their way to a carriage Dominique had waiting for their arrival. Vanitas waits outside with Chrysler and broods over what he's learned about Noé, while Dominique changes in the carriage with Noé. It is here that Dominique voices her concern over whether or not Noé has gotten involved with the Curse-Bearers because he blames himself for what had happened to Mina years before - attempting to console Noé and ultimately allowing him to drink her blood as he had in their youth.
- The concept of alternative dimensions has been used by Jun Mochizuki before in the form of the Dimensional Plane of Pandora Hearts. Whether or not the two universes are connected somehow has yet to be explored.