High Magic is the kind of reality distorting magic that the Ashuri practiced regularly and many forms of this are known because their Elven slaves learned of how to do such things by watching them. However, outside the gods and similar entities no one is singularly powerful enough to use High Magic without making it into a Ritual.
Other kingdoms learned either by watching the Elves or developing certain concepts on their own. After the Cataclysm only the elves maintained some of their secrets to High Magic. No one else retains the knowledge and changes to the way magic flows means it’s become much harder for these to work. Many a Lich has devoted their undeaths work to recreating High Magic.
Modern Elven High Magic Rituals are the only type of High Magic to be known to work.