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It's a rinnegan, he's able to use his EMS technique through his rinnegan.
It's the same eye that itachi planted Amatersu into as well.
Madara used his EMS during the battle against Hashirama, leaving him with one eye.
His original ms that he implanted became the rinnegan before giving it off to Nagato.
Sasuke still had his brothers eyes, so in short:
Sasuke+Itachi+senju cells= one eye rinnegan
Madara+senju cells= two rinnegans
It's a rinnegan, he's able to use his EMS technique through his rinnegan.
It's the same eye that itachi planted Amatersu into as well.
Madara used his EMS during the battle against Hashirama, leaving him with one eye.
His original ms that he implanted became the rinnegan before giving it off to Nagato.
Sasuke still had his brothers eyes, so in short:
Sasuke+Itachi+senju cells= one eye rinnegan
Madara+senju cells= two rinnegans