AGT (Adventure Game Toolkit) was created by Mark Welch and David Malmberg, but is not under current development. The AGT pages at www.markwelch.com/agt.htm provide a good starting point.
The AGT authoring system consists of a compiler and an interpreter (see Jonadab's listings at www.bright.net/~jonadab/if/).
Quoting from the FAQ for the rec.arts.int-fiction newsgroup:
I really wish I could put more pros here. When AGT was first released, ages ago, it was an improvement over what little IF creation software existed at the time. However, there really is nothing it can do that Inform or TADS can't easily do, and unlike the Tier (i) systems, it is not expandable. That is a key point: in Inform, Hugo, and TADS, you can basically get it to do what you want, at least in terms of the internal world (if not multimedia output). This is not the case for AGT. It is poorly ported. And, though some claim it to be easy to learn, some find AGT source incomprehensible. You can write a good game in AGT. It's much easier if you just use a different system.
Read the complete AGT entry at www.plover.net/~textfire/raiffaq/FAQ.htm#:AGT (Adventure Game Toolkit) or the entire FAQ at www.plover.net/~textfire/raiffaq/.
The AGT version of "Cloak of Darkness" was written by Al Golden (email@example.com).