The two eidolons I can think of right off the bat are:

Fenrir

None (default attack) - Earth element attack called "Terrestrial Rage"

Maiden's Prayer (found in Ipsen's Castle) - Wind Element attack called "Millennial Decay"


Carbuncle

None - Ruby Light - Casts Reflect on entire party
Emerald - Emerald Light - Casts Haste on entire party
Moonstone - Pearl Light - Casts Shell on entire party
Diamond - Diamond Light - Casts Vanish on entire party


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Also, for Dagger's eidolons, the more of each gemstone you carry in your inventory, the stronger each corresponding eidolon's attack will be.

For instance, if you have 99 garnets, Bahamut doing a full-amination attack will kick ass.

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Good luck! ^_^