What is the fine-grained topic of the following text: Mages automatically have fireball, but the other human spells that can be upgraded or “researched” for mages include flame shield (surrounds a unit with fire, causes damage to any units touching the fire), slow (slows down units drastically), blizzard (rain ice on enemy units and buildings), invisible (make your unit invisible until it attacks or wears off), and polymorph (turns an enemy unit into an animal). Only return the topic.
Upgradable spells for mages in a game