Charybdis was an IRCv3 server designed to be highly scalable. It implements IRCv3.1 and some parts of IRCv3.2.
It was meant to be used with an IRCv3-capable services implementation such as Atheme or Anope.
It is no longer maintained due to the scope of refactoring needed to support modern IRCv3 features.
These days, you should use Solanum instead. For almost all real-world deployments of Charybdis, it is better.