Invader is an open source modding toolkit for Halo 1, supporting multiple editions depending on the tool. It aims to replace tools of the HEK with an emphasis on tag data correctness and validation. Most of its tools are command-line operated like Tool, although invader-edit-qt is a GUI tag editor similar to Guerilla and Six Shooter is a GUI wrapper for some invader tools.
Familiarity with the command-line is a prerequisite for most of Invader's tools if you are not using Six Shooter.
See the Child pages menu for a complete list of tools included in Invader.