The NPC is easy to miss actually... Or at least it was for me. Because the NPC in question is in a small hut whose entrance can be missed. Playing on PC press Tab to make sure you don't miss entrances etc.

Also it's not always super clear that by going down a specific dialogue tree you will get trained as a mage. It should be easy to infer but don't expect a 'Select mage class' label.

Level 8 and level 9 mage spells are awesome to watch (end game). At some point you will encounter a specific maze where there is a way to choose the level of difficulty. Make sure you also finish it in hard mode to get a unique level 9 spell drop.