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The reason Tethys, the Great Oasis Spirit, had appeared was not just because Agua prayed to her but also because Khepri had called her through a Spirit Call, a powerful spell spirits used to communicate with one another across long distances.
The conversation she had with her last night likely helped Tethys gain some confidence or perhaps a newfound resolve, allowing her to step out of her comfort zone to protect the Oasis and the people she protected.
However, her power could only manifest temporarily through Agua's "Great Spirit Descent" Ability as her Priestess, so she quickly disappeared after helping them conjure such a game-changing spell as that one.
Now, the entire nest of the ants, a gigantic blazing labyrinth with several layers going underground, was flooded, frozen, and left on the verge of collapse.
Ninety percent of all ants likely died or were so weakened they probably couldn't even move properly now.