Summon an aeon in Overdrive!
X Grand Summon appears as Yuna's Overdrive. It allows her to immediately summon an aeon into battle with a full Overdrive bar, allowing them to immediately use their most powerful abilities. Yojimbo is an exception as he has no Overdrive, but summoning him with a full Overdrive gauge increases the chance he will use Zanmato.
Dissidia012 Grand Summon appears as Yuna's EX Mode. When activated, Yuna will use multiple aeons in battle with a Bravery Attack, and gains the EX Burst "To the Farplane", where each of her aeons will be used in battle against one enemy.
FFRK Grand Summon is Yuna's default Soul Break which at the expense of one Soul Gauge segment summons Valefor which deals non-elemental damage to all enemies equal to 110% damage potency of a convenient damage spell, calculation dependent on her Magic.
Evocation is the act of calling upon or summoning a spirit, demon, god or other supernatural agent, in the Western mystery tradition. Comparable practices exist in many religions and magical traditions and may employ the use of mind-altering substances with and without uttered word formulas.