Cassandros is a confusing character, it's actually a camera that is both a piece of meta and an account in the digital universe. It has the ability to control the mind of anyone who owns it, when someone owns Cassandros, they sort of 'become Cassandros'. Currently the girl who owns Cassandros is Alexandria.
Cassandros has created its own pocket dimension between the two universes, it can bring things in and out of it as it pleases. This allows it to bring things from the digital universe into the physical universe and vice versa. Cassandros does not like to do this, however, so if you somehow were to get into its dimension, the chances of you leaving are very low.
Alexandria took a picture of her family with Cassandros, this made the entire family get mind controlled all at once. Cassandros can really only control one person at a time, since the whole family got controlled, the camera had the family fight each other to the death. Alexandria killed her own father in the process and won full ownership of Cassandros.
After the Ascension Battle, Yenne acquired Cassandros, and is its owner. It is unclear what he plans to do with Cassandros and it's been theorized that Cassandros is in a forcefield much like Providence is. This may be what is preventing Cassandros from controlling Yenne.