Invader is an open source modding toolkit for Halo 1 which serves as an alternative or complement to the tools of the HEK and H1A Mod Tools with an emphasis on tag data correctness and extending limits where safe. It supports all editions like MCC, Custom Edition, Xbox, PC, and Demo but does not support tag extensions from OpenSauce.
Familiarity with the command-line is a prerequisite for most of Invader's tools if you are not using Six Shooter and invader-edit-qt.