I made a mod for The Binding of Isaac, and many more people downloaded it than I ever expected. It’s currently at 2,449 subscribers on the Steam Workshop. For a few weeks, it was on the front page of the game’s Workshop page and even a few YouTubers made videos showing Gameplay of the characters. One of the videos has over 200,000 views.
The mod contains two custom characters, each with their own custom items, the dart monkey and sniper monkey from Bloons Tower Defence.

To make the mod I had to make and modify a lot of spritesheets for things such as the character’s movement, shooting, interacting etc. I also had to make sprites for the character select, loading and death screens. The items also involved making new sprites for the item itself, its projectiles and other effects.
Most of my time was spent learning the LUA language and how it integrates with the game so that I could change the character’s appearance, stats and starting item. It also allowed me to create custom items that give the player different stats and change their appearance, stats and even render a weapon sprite that rotates when the player shoots.

The mod can be found on Steam here: https://steamcommunity.com/sharedfiles/filedetails/?id=3017635979