From IFWiki

Phoenix is an operating system used by some IBM mainframes.

Phoenix is also the name of a particular computer that ran the Phoenix operating system at Cambridge University. Acheton, the first major text adventure developed in the UK, was developed on this machine. Between 1978 and 1985, the programming language and interpreter used for Acheton were extended and used to create more games, collectively called "the Phoenix games." The Phoenix development system is probably the first text adventure system to be used by people other than its original authors. Some of the Phoenix games were later ported to home systems and published by Topologika. Others have more recently been converted to Z-code.