Clarent / Clarent Blood Arthur

Radiant Sword of the King
Rebellion Against My Beautiful Father. Together, they are Mordred's Noble Phantasm. It was originally a sword stored in King Arthur's treasury. But Mordred broke into the treasury when she caused her rebellion. Perhaps being interested in it as the “sword that symbolizes royalty,” she came to favor using it.
It was originally a B rank sword bestowed during the succession of kingship, used to amplify the king's authority… the king's royal aura. Concretely, it raises the owner's physical stats by one rank, and grants and raises the skill “Charisma.” However, since Mordred was never acknowledged as king, the sword itself is lowered by one rank and she cannot acquire any bonuses while wielding it.
However, the ability of “amplification” itself has not been lost, so Mordred drives her excessive hatred into the sword in the form of mana, thereby amplifying it and allowing her to shoot it as an attack.
To make it easy to understand, “Clarent Blood Arthur” is an application of “Mana Burst,” and the red lightning that it manifests are her twisted feelings towards her father that have been amplified by the sword.
In the light novels, her discord with the King of Knights was (one-sidedly) resolved, but she can still shoot it regardless of her feelings towards her father. In that regard, she possesses the innate talent inherited from the blood of the King of Knights.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia