The only condition for using the Calydonian Boar was to be filled with hatred, as the hide was essentially a manifestation of hatred and desire, the essence of the Calydonian Beast.
For a pure saint like Jeanne d'Arc, wearing the boar hide would have no effect, but Shinji was different. The sacrifices of Justeaze and Artoria had already filled his heart with hatred. The calm he had shown earlier was merely him forcibly suppressing his emotions to maximize the power of the Calydonian Boar, even stronger than Atalanta herself wearing the hide.
Shinji didn't know whether he would become a beast or a demon after wearing the boar hide, but one thing was certain: his attributes like attack, defense, and agility would dramatically increase, and his vitality would become extremely tenacious, allowing him to relentlessly pursue his object of hatred.
Combined with Spartacus' Noble Phantasm stored in his spiritual core, it was no exaggeration to say that Shinji wearing the boar hide would be even more formidable than his previous three-headed, six-armed form. Such a monster would be a severe challenge even for top-tier Servants.
More terrifyingly, this monster combining two major Noble Phantasms could self-detonate once more, and this final self-detonation would surpass all previous ones. This time, it would be a true fantasy collapse of both the spiritual core and the Noble Phantasm, comparable to the last arrow "Stella" of the great hero Arash, capable of blowing up the entire mountain.
This was Shinji's true trump card and the secret agreement he had made with Atalanta.
The agreement was simple: if the massive explosion near the Greater Grail didn't destroy either the Greater Grail or Amakusa Shirou Tokisada, then lend me your boar hide. Hence, Atalanta shot the arrow to deliver the boar hide to Shinji.
"I will kill you, at any cost."
Holding the boar hide above his head, Shinji finally stopped suppressing himself, letting all his emotions explode.
At this moment, all his feelings for Justeaze and his bond with Artoria transformed into hatred for Shirou.
Sensing this almost tangible dark emotion, the black hide "came to life," emitting black mist from its surface, twisting and gradually wrapping around Shinji's body.
Seeing this, Shirou regained his composure and said to himself:
"You scared me. So that's why you couldn't sense your true power? Hatred blinds people, indeed. Not that I, the instigator, have any right to say that... Although I'm curious to see what kind of monster his hatred will create, I'll pass. It's too risky; one wrong move and it might not be reversible. So—it's your turn, heroine, to save the world."
Before he finished speaking, a crack appeared above the Greater Grail, and a pure, immaculate white light flowed through, directly entering Shinji's body.
"How much benefit you reap next depends on your fortune. Ha~ After fighting for so long, I'm a bit tired, time to rest, rest..."
Shirou yawned lazily, tossed his sword aside, and lay flat on the ground.
However, before lying down properly, he suddenly jumped up.
"This presence... Not good, Semiramis is in danger. Command Seal, Command Seal—wait, this feeling is... No way, this is Earth, when did she follow us here? Hey, old jewel, quickly isolate the Throne Room, or we'll be facing a planetary-level enforcer."
"It's you who will face her, not us," the old man's calm voice came from the crack.
"I'm just an observer."
"Are you sure?"
Shirou raised an eyebrow.
"Let me remind you, if either she or I have any problems, the other side will be more insane than this guy."
The old man remained calm: "What problems could you have? You are merely a consciousness projection through that device, and she should be using a similar method. Even if I have problems, you won't."
"What you say makes so much sense I have no retort."
Shirou's mouth twitched.
"So I've decided to stir things up."
"Don't you think you've caused enough trouble?"
"Such small matters don't befit your status or that of the big shots outside. Let me see how to make it more interesting... Got it, that sword was forged to deal with a certain cute girl back then. Now that the sword's true form has appeared, how about inviting that girl to show up as well? Let me see, let me see, my permissions are just enough to recreate that mythological battle, which also matches your status and grandeur."
"..."
This threat finally broke the old man's usual calm.
"Do you want to destroy Earth's civilization again?"
"Not that extreme. To destroy civilization, that girl would need to reach the seventh stage. She's only at the first stage when just released, which is relatively manageable. Plus, with that Holy Sword around, at most, it would be a localized anomaly—unless I place her where that big spider is sleeping."
"..."
"Wait, if I remember correctly, the three Counter Force back then caused trouble for the entire solar system. The concept of U.O might have been created later to deal with those three Counter Force. So, letting her find the big spider seems quite fitting."
"Can I curse?"
Hearing the old man's loss of composure, Shirou smiled.
"Go ahead."
"Fine, you win."
The old man was silent for a moment, ultimately refraining from cursing. With a sigh, an unknown, secret ripple expanded outward, instantly shrouding the Throne Room in a phase sense.
"Thanks, old jewel. Now I don't have to worry about things getting out of hand," Shirou said as he lay back down, "Sorry for the trouble again."
"It's nothing, you've caused me trouble more than once or twice. Just because I can handle it doesn't mean Matou Shinji can."
"If he has the guts, he can come beat me up. Honestly, it's been many years since anyone's tried. If he really can... it looks like he actually might. No, I need to hurry up and recover; getting beaten up shouldn't be too embarrassing."
With that, Shirou closed his eyes, and the spatial crack above the Greater Grail also closed.
Not long after, two beams of light, one red and one blue, shot straight into the sky.