A heropack is basically a player made extension for the fiskheroes mod that can add new suits, and alternate styles for base mod suits. The code for heropacks can be thought of as legos where they can pieced together in different ways. But unlike Lego, you can take apart the bricks. You can remove the studs on top, stick 'em into the sides of other bricks. You can do basically whatever you want. Even the main mod itself is built as a heroPack too, so everything the mod can do you can do.

1. download the LATEST version of Fisk's Superheroes Mod:
link: https://www.planetminecraft.com/mod/fisks-superheroes/
(at least open the game once if it's your first time playing the mod)

2. download the heropack and place it at this location:
AppData\Roaming.minecraft\fiskheroes
(no need unzip)

3. enjoy!

Heropacks are made for the fiskheroes mod which is made for 1.7.10, often confused with 1.17 (edited)

Use the mod as a template by extracting it and join the Heropack Central Discord for help!