What is the source of the Mages’ power?

I've never played a full game against a human opponent, only against the AI, so my next observation is for playing against the computer. In the early stages, mages are weak, but when the Oracle hero has spellpower plus leadership and luck/pathfinding, she becomes a monster. She has a tactic against any army, using frenzy and enslave on some, clones on others, and Armageddon on others... Also, you can leave a second hero to guard the castle – the Oracle will pass on the necessary experience through mentoring... All neutral creatures are either defeated without losses or join your side... The Oracle can carry titans or titans and rakshasa with her... I definitely defeated (the main hero) a dwarf, a warlock, and even a necromancer this way... You only have to get creative if the enemy has immunity to dark magic... and with enslave, you can defeat anyone... and of course, you need to remember about artifacts that greatly enhance creatures...