But the effect of the Chaos Phantom Demon is to first offer control to the opponent, then "banish all monsters on your field from the game."
During the resolution of this effect, the control of the Chaos Phantom Demon is Takuma Saiou. So from the Fool's perspective, this is an ally, not subject to "opponent's effect."
The Fool can only be immune to ally firepower when in the upright position, so even the mighty Fool can't stop the Chaos Phantom Demon's ultimate move, getting directly banished to the Other Dimension, disappearing without a trace.
Naturally, such an unscientific effect of the Chaos Phantom Demon was removed when it was made into a physical card back then. However, years later, the official added a new variant of the Chaos Phantom Demon card, called "Chaos Phantom Demon · Amiteel - Void Phantom Roshamone", with the skill inscribed in the card name, and perfectly replicated this unscientific effect from the animation.
The effect of this physical card really is to give its control to the opponent, then banish all monsters on the controller's field from the game.
This behavior of first infiltrating enemy forces, pretending to be allies, then opening fire is truly insidious. Many similar full-resistance bosses that are "unaffected by opponent's effects" like the Fool can't withstand it. Including that Ammon in the animation, also a full-resistance boss "unaffected by opponent's effects" was tricked by this move and died with regrets.
Hmm, just like Saiou now.
Looks like he's about to lose his mind.
"Impossible! Impossible!"
Saiou continues to rage.
"My Master of Light possesses the power to control all the light in the cosmos!"
"My Fool is the invincible ultimate shield!"
"My destiny is absolute!"
"This is impossible! It's all fake! All fake..."
Mizuchi: "..."
"Of course, the Chaos Phantom Demon will return at the end of the turn, and I can't deal any battle damage to you this turn... but that's alright. There's still your World."
Yugen continues with a smile.
"At the end of the turn, first resolve the effect of 'Sacrificial Lotus' in the graveyard, special summon 'Sacrificial Lotus' from the graveyard."
[Sacrificial Lotus, Attack Power 0]
"Then it's the effect of 'Chaos Phantom Demon · Amiteel'. At the end of the turn, control returns to the original holder.
Come back, Amiteel."
The Chaos Phantom Demon responded vaguely, having eaten its fill, cleared the opponent's field completely, then patted its behind and flew back to Yugen's side.
Saiou: "......"
"Lastly, it's your monster, the effect of 'Secret Ceremony Power 21 - World'. Use 'Yellow Springs Frog' and 'Sacrificial Lotus' as offerings, skip the opponent's next turn."
Yugen waved his arm.
"Zawaludo (World)!"
In the solidified space-time, the dial turned swiftly, and in an instant, it was once again a new turn for Yugen.
"Alright, your turn is skipped, now it is still my turn.
My turn, draw a card."
Yugen waved his hand.
"It's been a good duel, but it's about to end here.
Activate the effect of Chaos Phantom Demon · Amiteel. Once per turn, if there are no monsters on the opponent's field, directly inflict ten thousand battle damage to the opponent player!
Chaos Annihilation!"
The final blow of the Chaos Phantom Demon, like the descent of the Demon God, an apocalypse blow.
Takuma Saiou looked up at the incoming Chaos Phantom Demon, as if he could truly feel the power to annihilate the cosmos contained within.
It seemed like, at this moment, his cosmos, his destiny, were destroyed under the strike of the Phantom Demon.
[Saiou, LP 900→LP 0]