Regarding why Sakatsuki took Fran's Noble Phantasm, the first thing to explain is its characteristics.
[Bridal Chest]
[Rank: C]
[Type: Anti-Unit]
[A warhammer wreathed in branching electrical discharges. Normally used as a blunt weapon, the spherical head contains her heart, so she carries it with her at all times even outside combat. It efficiently collects and stores leaked magical energy from herself and the surroundings. During battles where large amounts of residual magical energy are continuously generated in the environment, it simulates a "perpetual motion machine of the second kind" by coordinating with the electrical currents.]
Paired with this is one of Fran's skills:
[Galvanism: B]
[Freely converts bioelectricity and magical energy, as well as stores them. Can instantaneously transform intangible attacks like magical light, wind, or bullets into electricity, discharging them to neutralize threats. Additionally, the body strengthens according to stored electrical charge, and damage recovery accelerates. This skill embodies the galvanism concept interpreting life activities through bodily electric currents, falling under alchemy's domain alongside phlogiston and aether.]
—As Fran's core, this skill is anchored to Bridal Chest. Thus, by obtaining Fran's Noble Phantasm, Sakatsuki would inherently possess this skill too.
As for how Sakatsuki can appropriate other Servants' Noble Phantasms, it stems from his 'legend.'
His initial power was inherited from the Great Hero, and throughout his journey, he continually assimilated abilities and weapons from other heroes and warriors. One could say Sakatsuki himself grew and evolved within the radiance of heroic legends.
Much like Astolfo, whose adventurous exploits allowed him to receive Achilles' gifted Noble Phantasm, Sakatsuki could similarly inherit records from other Servants.
Amidst golden light that flickered and faded, the maiden clad in white bridal attire withdrew from this Holy Grail War. Yet her heart—her Noble Phantasm "Bridal Chest"—transferred into Sakatsuki's hands, morphing into an unstable orb of light.
"Is it disintegrating due to the master's disappearance?"
Sensing the dissipating particles, Sakatsuki calmly clenched his fist. Platinum-hued miracles surged through his arm, gradually merging into the luminous sphere.
Touch the core, rewrite its authority.
With Fran's lingering will, Sakatsuki effortlessly claimed the Noble Phantasm. However, as the warhammer's form slowly materialized anew, he frowned in dismay.
"How am I supposed to use this...? Should an Assassin really go around smashing people with a warhammer?"
The mental image was too absurd to contemplate.
As if sensing his dilemma, deep within his unseen soul, an ancient and mysterious tome emitted a faint glow.
Then, under Sakatsuki's astonished gaze, the warhammer dissolved back into vast imaginary spiritrons. Guided by an unknown force, they condensed and coalesced, transforming into a golden bead strung on a red cord that gently settled around his neck.
[Servant Status Updated]
A familiar notification echoed from distant reaches. Closing his eyes, Sakatsuki processed the information flooding his mind.
[Noble Phantasm 'Bridal Chest' and Innate Skill 'Galvanism (Incomplete)' recorded. Fusion complete. New Noble Phantasm obtained: Electromaster]
[Electromaster: B]
[By analyzing the genius creation of Dr. Victor—the internal circuits of Frankenstein—this new Noble Phantasm was obtained through his legacy.]
[As a power leaning toward the scientific side, it allows the wearer to harness flowing electricity (electric charge). Though it cannot compare to 'Human Myth·Lightning's Descent,' if the wielder possesses ample knowledge and inversely proportional hair volume, it might unleash unbelievable effects, no?]
"Such profound malice toward STEM nerds..." Even in his Assassin state, Sakatsuki couldn't suppress his urge to complain." "And this is a Light Ball (Pikachu's signature held item in Pokémon), right? This is absolutely a Light Ball?!"
"Should I shout 'Thunderbolt' or something?!"
As Sakatsuki vented, the powerless 'Theater' gradually dissipated. With Caster's barrier gone, the information of Frankenstein's defeat immediately reached her Master—Caules.
"Damn it!"
In front of all the Masters, Caules slammed his fist against the stone wall with such force that blood immediately seeped from the scraped skin. The stinging pain was drowned out by the raging heat in his mind.
He shouldn't have shown such a pathetic side in front of other magi, but this time, his emotions completely overwhelmed his reason.
Without warning, without orders, without consultation or aid—he had personally sent Berserker to her demise at the enemy's hands!
He should have contacted Berserker the moment he noticed Sakatsuki's suspicious movements! He should have used a Command Spell to summon her back the instant he realized she had stopped moving!
Blood trickled over the back of his hand, where the three Command Spells burned like a mocking brand, stinging his eyes.
"Calm down, Caules. It wasn't your mistake."
Perhaps sensing her brother's anguish, Fiore tried to console him. But Caules shook his head vehemently, shouting:
"No, it was my mistake! I knew the situation and still chose to gamble! It was my fault. If I had just used a Command Spell in time... If I had awakened her, it wouldn't have ended like this! I wouldn't have let her... I wouldn't have let Berserker die in vain!"
It was my miscalculation, my misjudgment—Caules sank into deep self-reproach. But from Fiore's perspective, his words weren't entirely accurate.
His mistake was an unavoidable one. Servants were the main force on the battlefield, and Masters hiding behind them shouldn't micromanage their decisions. If he started recalling his Servant at every unexpected turn, Caules would only be walking the opposite extreme of Gordes' folly.
"At the very least, we've learned about the nature of Red Caster's Noble Phantasm, haven't we?"
It wasn't a meaningless sacrifice, Fiore thought. But Caules, as a magus, shook his head in denial.
"But we still don't understand how Caster's Noble Phantasm works... Don't worry about me, Sister. Focus on the battlefield instead."
"But—"
"Leave me alone."
Hearing Caules speak in that uncompromising tone, Fiore had no choice but to refocus her attention on directing the battle operations. Left alone, Caules leaned against the corner of the room, covering his face with his hand, remaining silent for a long time.
The regret of a companion (friend) suddenly departing—this young boy would need considerable time to come to terms with it.
Meanwhile on the battlefield, having barely shaken off Spartacus, Vlad III and Astolfo—who had abruptly received news of Berserker's elimination—now found Sakatsuki still within the forest.
"You... you!"
The pink-haired paladin, riding his hippogriff, roared as he dove toward the young man with unprecedented seriousness, thrusting his lance forward:
"How dare you! How dare you kill that child! Damn you, I'll never forgive this!"
"To slaughter my brethren so... my mortal enemy, are you prepared to face my wrath?"
The very earth seemed hostile to Sakatsuki as stake after stake erupted from the ground. Vlad III advanced with a dark expression, surrounded by countless stakes, radiating murderous intent.
Meteors from above, muddy currents below—a deadly encirclement closed in on Sakatsuki from all directions.
Facing the Black Faction's vengeance, the young man merely raised his head as the golden orb on his chest began glowing brilliantly.
"Though this goes against my usual character, there are some things I absolutely cannot compromise on. For example—"
"Pikachu, use Thunderbolt!"
In the next instant, golden lightning illuminated the entire forest!