Greatswords before the 15th century saw little use, since, well they did not have much use. They're big, unwieldy, and you still can't cut through metal armor (be it mail, or a helmet, or something else). So you ended up using them like giant clubs, and then what's the point?
The reason why they became more used in the later 15th century and especially in the 16th century is because of the meta of pike formations. Greatsword usage would be to swing them in circles in front if you to knock pikes away and disrupt the formations, and once that was done you drew your sidearm and started actually fighting.
You should have added "in Europe".
Because 2h swords had their use way before 15th century.
For example: https://en.wikipedia.org/wiki/Zhanmadao BC anti cavalary 2h sword
https://en.wikipedia.org/wiki/Changdao elite infatary 2h sword used to break enemy formations
Also, it is really implausable to assume such sword could have existed in, say, Viking Age? Dane axes had simillar application to this Changdao - to break shield walls using advantage of reach and high relative power. It was however way cheaper and more practical to forge long axe or poleaxe than to cast such a long blade. But in Calradia, there might have been different factors in play.