Eternal Arms Mastery (無窮の武練, Mukyū no Buren?) prevents degradation of fighting skills when under the effect of mental hindrance.[1]

Rank User(s)
A+ Lancelot (Berserker), Lancelot (Saber)
Mastership of combat arts has reached the point of being said to be unrivaled in one's era. By complete merging of mind, body and technique, it is possible to make use of full fighting skills even when under the influence of any sort of mental hindrance.[1]
Minamoto-no-Raikou
The dexterity of combat arts performed to the extremes. Prevents the degradation of fighting skills no matter the kind of circumstances, even if she loses her armaments, etc.
B Melusine (Lostbelt)
A skill copied from Heroic Spirit Lancelot of the Proper Human History. Regardless of one's mental state, they will be able to fully display the combat techniques they have mastered. Combat experience that has been engraved into one's body from excessive training...is what could be said about this, but it's a skill that isn't that necessary for Melusine who was born strong. Melusine hates the existence of this skill. Creatures who were strong since birth do not need techniques.

Variations

Servant Skills
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