Have you ever been on a server with your friends trying to farm this one boss for a specific drop, but friend A is out mining, friend B is on the other side of the world fishing, and friend C is afk at spawn? Well I certainly have. This makes it a huge pain to kill any decently difficult boss because of just how much longer it takes. So I created this mod to deal with the issue.
When a boss spawns, it checks to see how many players are within a certain area of its spawnpoint and scales its HP based on that number instead of the total number of players in the server.
There's a very obvious issue with this approach; it allows players to cheese bosses by spawning it with one player nearby, then having everyone else fight the boss together after it was spawned. I was thinking of ways to try to prevent this, but then I realized that in vanilla Terraria you can do the same by spawning a boss, then having everyone else log in to the server afterwards. So if you really want to use this mod to cheese bosses, be my guest. But at that point, why not just find a mod that disables multiplayer boss scaling altogether?
This mod likely has many bugs I've yet to discover, and while I'm not sure if I intend to actively maintain it or not, feel free to open an issue or a pull request and I'll gladly look at it.