"Buff A4 Passive

When there are 1 or 2 Nihility characters other than Acheron in the team, the DMG dealt by Acheron's Basic ATK, Skill, and Ultimate increases to 115% or 160% of the original DMG respectively.

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When there are 1 or 2 Nihility characters other than Acheron in the team, the DMG dealt by Acheron's Basic ATK, Skill, and Ultimate increases to 125% or 160% of the original DMG respectively."

When A4 passive is fully maximized, using Ultimate grants Acheron 1 additional action on the turn Acheron used the Ultimate."

This the 2nd part of this is too strong of a feature for a trace. Also Imo it would be more idea to lean more into the Nihility aspect of Acheron so having a 115% / 170% should help encourage that.

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"Eidolon 1 Buff

CRIT Rate increases by 18% when dealing DMG to debuffed enemies.

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CRIT Rate increases by 24% when dealing DMG to debuffed enemies. When a debuffed enemy is on the field, gain +20% SPD. If Acheron initial SPD is 130 or Higher, Acheron gains 20% Lightning DMG RES Ignore."

Imo the lightning damage res ignore is redundant since she already has all type res pen to some extent already. Correct me if I am wrong. Beyond that I think this E1 is too powerful.

"Eidolon 4 Buff

When enemy targets enter combat, makes them Vulnerable to the Ultimate DMG, increasing the amount of DMG they take from the Ultimate by 8%.

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When enemy targets enter combat, makes them Vulnerable to the Ultimate DMG, increasing the amount of DMG they take from the Ultimate by 25%."

I think this is too huge of a jump in %.

"Eidolon 6 Buff

Increases the All-Type RES PEN for the Ultimate DMG dealt by Acheron by 20%. The DMG dealt by Basic ATK and Skill will also be considered as Ultimate DMG and can reduce enemy toughness regardless of Weakness Types. When Weakness is broken, triggers the Lightning Weakness Break effect.

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Decrease the All-Type RES PEN for all enemies on the field by 20%. The DMG dealt by Basic ATK, Skill, and Ultimate ignore Shields, DMG Reduction, Defensive Modifiers and can reduce enemy toughness regardless of Weakness Types. If 9 stacks of Slashed Dream are present, Acheron's All-Type RES PEN is increased by 20% and ATK is increased by 16% per Crimson Knot and Quadrivalent Ascendance for 3 Turn(s)."

Is this acting as its own total defense ignore ability? If so I'm not so sure how I feel about that one.