Minecraft Boss mod, with many bosses?

Find a boss mod in Minecraft version 1.12.2 and the. Bosses should drop keys if possible.

Heard of a friend of such a mod, what's his name?

This is also very easy without mods. Go to minecraft.tools/de/spawn.php. Sat you can generate your mob (end dragon or whoever you want), you can also set what it should drop and much more! Before you run to the mods, try commands.