WARNING: THIS PLUGIN IS MADE FOR 1.20 AND IT EXECUTES COMMANDS AT THE CONSOLE SO IT MAY NOT WORK NORMALLY ON VERSIONS BELOW 1.19

The timing function requires the plugin CommandTimers and you can download it from https://www.spigotmc.org/resources/commandtimers.79104/
After installing CommandTimers, make sure the timers are enabled. Then do the command "/commandtimers every 1m bosssystem activate" in order to let the bosssystem generate the custom mobs in wanted time periods.

Commands: (You can also use the /bs command)
/bosssystem reload - reloads the config
/bosssystem activate - activates all custom bosses in your config
/bosssystem help - show the help list

You may also use the /forceload command in order to keep the wanted chunk in progress.

This is my first functional plugin (*/ω＼*) so there may be a teeny-tiny bit of issues.