What is Atlantis?
Beginning with the earliest MUDs (Multi User Dungeons), on through today's MUSHes, MUXes, MOOs, MUCKs and all the other varied offspring of the original MUD, there have been text-based multiuser games around since the early days of the Internet. But to connect to these MU*s, you generally need a client slightly more advanced than plain old telnet!
Atlantis is a client specifically designed to aid in playing those type of games. It's been around since macOS 10.3, and has been updated to 64-bit support for Catalina. There are still a few quirks, however; notably, the scripting system no longer works due to many myriad changes in how Apple handles Perl (and the untimely death about six years ago of the friend who wrote the CamelBones library Atlantis' scripting was based on), and the app does not yet support dark mode properly. (Or at all.)
It's in the midst of a rewrite to get it redone to modern APIs, and I may end up open-sourcing it. But in the meantime, at least there's a current build! (And a website that doesn't look like it's from 2006 anymore.)
What can Atlantis do?
In addition to the expected feature-set of every MU* client — connect to a game, provide highlighting, log output and so on — Atlantis has a number of fairly advanced features. You can find a few of them in the sidebar right on this page.
That's all cool, but why call it Atlantis?
As these games are often known as 'MU*,' the Greek letter 'mu' seemed an appropriate emblem for Atlantis. And given that the great sunken civilizations were Atlantis, Mu and Lemuria? The 'Mu' client got named Atlantis, and the underlying window toolkit was named Lemuria.
(Yes, the developer is a dork. Cope.)