First download HAC2 from
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INSTALLATION
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To install HAC 2, simply copy "loader.dll" to the relevant 'controls' folder.
For example, for Custom Edition the file should reside in:
C:\Program Files (x86)\Microsoft Games\Halo Custom Edition\controls
For Halo PC:
C:\Program Files (x86)\Microsoft Games\Halo\controls
Run the game once so that it can create the HAC folder.it should be in "Halo CE: C:\Users\Your_Username\Documents\My Games\Halo CE"
Exit the game and open the HAC folder and make a new folder called "packs"
Download any one of these (or all of them) and put them in the packs folder without extracting.
Halo 3,
Halo Reach and
Halo 4.
Create a shortcut for halo on your desktop and go to properties and add -console in the end of the target apply it and launch the game.
Press ` and a console will open.Type "optic load halo4" or any other pack you want to load.