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Even though she is an Assassin by nature

Even though class-wise she is a Saber, since she is an Assassin by nature

Because Achilles was not summoned as a Lancer, the spear has lost some of its abilities, such as a curse of incurability on the wounds it inflicts.

This weapon that possesses the destructive power to paralyze the world,
despite having been antagonized by Arjuna shooting with his bow at full power, left ravages that were said to turn the whole surroundings into a barren land for twelve years.
Although it’s registered as a Noble Phantasm,
Ashwatthama will not use this Noble Phantasm.

Caenis explains that Poseidon was from the same line of ships as Demeter, which means that to the people of the past, he too was a kind of earth god. As such, she can naturally use the authority of the earth.

Musashi is a bit sad that she wasn't more helpful during the fight. Even having reached "zero" with the sword, it seems there are some things that still cannot be cut

 a "finite" sword that strives to reach only a single "correct answer". To shave one's existence away to the extremes until there is nothing left, and yet to cut everything until only "something" still remains. That is Musashi's "Zero". A sword that establishes a "conclusion" that not even the gods can escape from

The Kintoki AI takes over and says that they have one last resort to defeating Zeus - a large summoning ritual which calls on the biggest names in the history of humanity!

He says that's it's similar to Chaldea's summoning, a system for summoning all kinds of Heroic Spirits from all across time. By nature, a kind of chain summoning is likely to occur within lostbelts and singularities. The idea is to give it a direction. This is the Great Summoning System! It's official name is---

Godbreaker Spell Protocol - Crown Designated Heroic Spirit Summoning

 in order to ensure that someone who would normally be summoned doesn't appear, you need a mechanical device called a "Great Summoning Device".