THIS IS MY PERSONAL THEORY.
This theory is largely unfounded, but sod it.
Majora started the race for the Triforce that we see in the HH and the
TP prologue, giving the fused shadows to those who had the best chance (the Twili). After they failed, he launched a full-scale attack on the world (as seen in the MC prologue). However, they were defeated by the Minish and the hero, and Majora himself was sealed away in a mask in Termina whilst his demons were sealed in a chest.
Majora is the son (well, creation) of demise.