Mob spawn when player get close to block

So basically im on about to finish the next update of my mod, but im stuck with a boss, i want to make so that ;

If the player get close to a custom block, it will destroy the block and then spawn the boss.

Kinda lile how the twilight forest boss spawn, please i really need help, and thank you in advance(if i could get an image of the procedure it could be nice)