The lore is simple, and she'll probably explain a lot since she's changed the system a lot, but I can give the super quick rundown.
-Mages cast different types of magic, and all can summon spirits of dead heroes. (it'll probably be limited but I'm not GM so I can't say what will pass and not)
-These dead heroes they summon are Heroic Spirits. From a conversation we had the spirits will be 18th century, no later.
-Heroic spirits typically are of heroes. EX: King Arthur from Britain, Princess Medea from Colchis in Greece, and Gilgamesh from Babylon.
-While my examples are from the show, feel free to get creative. Also, just remember that not all Heroic Spirits are well... heroic. They have their own motivations and in some cases are totally twisted. What's important is whatever criteria Ambra sets for these spirits.
-From a conversation Ambra and I had, if the mage dies so does the spirit. Also works other way around.