Any custom mob can be marked as a boss by setting the boss flag to true. The spawn mechanics remain the same. The main difference between a boss and a regular mob is that one bloodmoon can have only 1 boss.
Using the boss bar
As soon as the boss has been spawned, the boss bar (if enabled) will show the name of your boss. The bar itself will also show the total health of the boss. When the boss is getting damaged, the bar will drop.
How to find the boss?
There are several methods of finding the boss. The most difficult method is luck. The easiest solution is by using the "Pathfinder".
What is the pathfinder?
The pathfinder is an item using the compass material. While having the compass in your hand, if you right click, the compass display a path to the boss. The compass itself will also point towards the boss. By following the trail, you will find the boss.
Retrieving the pathfinder
The pathfinder can be retrieved by typing the following command:
/bm item give pathfinder 1 mrgeneralq
In order to use the pathfinder, you will need the following permission:
bloodmoon.compass #permission to make compass point towards boss
bloodmoon.pathfinder #permission to make trail become visible