The First Spinjitzu Master origin theory:

So this is my theory about the as of yet unknown origin of the first spinjizu master. A lot of this comes from Season 5, so if you haven't seen it and want no spoilers, what are you doing, stop reading this, stop it I say. Alright, here we go: We are told in season 5 that there is a realm crystal, which can take the person who holds it to any of the 16 realms that exist. We are also told that the first spinjitzu master had this realm crystal. From the start of the show, we are told that he created Ninjago, and that essentially there was no other people around when he created Ninjago. Yet he had 2 sons, and there are a lot of people living in Ninjago. In season 3 we find out that he created the serpentine, so he could have created other people too I guess, but this is what I believe: The First Spinjitzu Master was born in one of the other 16 realms. he obtained the realm crystal, and with it he traveled through the realms. Eventually he came across Ninjago, which was an empty realm upon his arrival. He forged the golden weapons and created Ninjago, as well as The Overlord unintentionally. Once he had split Ninjago in two, he opened up a portal to some other realm and let those people populate Ninjago. You could theorize that his realm was overpopulated/dying, and so he was trained to find his people another realm to live in. He most likely married on of those people, and had his two sons. This would explain how he existed "long before time had a name" with no other people, where all the people of Ninjago originated from and how he knew about the other realms, because he had traveled through them.