So, the Ancients were a race of inventors of unknown origin. They completed their best achievement: the Galactic Nova. They were celebrating their success, when a traitor emerged. That traitor was none other than Galacta Knight himself. He turned on them, planning to wipe out the entire race. He killed ALL of them, save one. Meta Knight. He created the Master Crown, a treasure that will seek out a hero and bestow him with great power. But Galacta Knight was one step ahead. He made it turn the hero evil when it was aquired. Finally, Meta Knight used the Nova to seal him up in a crystal prison, and entrusted the crown with the Landia sisters, who stood on top of Dangerous Dinner. He also put in the Lor Starcutter. He then fled to Popstar, where he pretended to be a "normal" knight. This is why he retreats when his mask falls off, he doesn't want to reveal himself as an ancient. But the crown sensed the purest heart of all: Magolor.
Return to Dream Land happens
Then, Meta Knight decides he wants to grow stronger. So he had his crew invent new training simulations based off of Kirby's adventures. But he does not want to fight guys like Marx and Star Dream, oh no, he wants to battle the STRONGEST WARRIOR IN THE GALAXY!
All the Meta Knightmare modes happen.
Then, he invents new sims for Kirby!
The Arenas and True Arenas happen.
TL;DR: MK and GK are Ancients, and MK made the crown and sealed up GK.