Only 2 summon is allowed to be in play at a time per summoner.. A character is REQUIRED to visit summoner temples throughout the universe.. Summoner Class can start out with one summon. Summoners can enchant amulets once every 30 years to be able to summon a creature.. Summoners start out with 3 summons of their choice, along with offensive/Defensive spells corresponding to the summon.
Summoning rules:
Summons are your allies, not weapons.. Summons have minds of their own, the summoner is just their anchor into this realm.
Summons are allowed to attack the summoner if the player breaks their pact (I.e treats the summon like a pet, rather than an ally)
Summons that aren't starters will be required to be acquired via temple.
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Corrupted Summons are made when a Summoner falls to the Dark Side, corrupting their summons as well as themselves.. Thus creating a Dark Summoner. See the tie-in?
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List of Final Fantasy summons:
http://finalfantasy.wikia.com/wiki/List_of_Summons
Subject: Summoning info