Lets start here:
Look at the part where Kabuto says about being impatient may cause the opposite effect of what Itachi really wants. Then it cuts straight to Madara.
That right there was foreshadowing Madara releasing Edo Tensei, and Kabuto KNEW that was going to happen. Wait, how?
We knew Madara lived until Nagato was young, he likely died around that time as well. Orochimaru probably just started to research forbidden jutsu and probably learned Edo Tensei then. So I'm fairly sure Madara and Orochimaru have met, and that is how Madara learned of Edo Tensei's seals.
Wait, didn't Madara learn it from Tobirama, the creator of the jutsu? I'm sure Tobirama has almost NEVER used this jutsu, maybe once to test it out. It requires a human sacrifice, and no one would allow a Hokage's brother/Hokage to use human sacrifices. Secondly, Tobirama or his brother made the jutsu forbidden for a reason, one shouldn't use forbidden jutsu, that's why its well... forbidden.
Another subtle hint:
Why in the WORLD would Kabuto call himself Tobi's assistant? Now, I maybe just speculating here, but does Kabuto have any allegiance to anyone besides Orochimaru? He might've very well known that Madara knows Orochimaru in some way, and he is the one who uses Edo Tensei, so Kabuto simply said I am his assistant.
This is complete speculation, but Madara is holding the back of his neck after a panel zoomed on Anko's neck, showing her Cursed Seal. That could very much be implying that Madara might have a Cursed Seal, this could mean only one thing: Orochimaru gave him it. Only Orochimaru has been shown to give Cursed Seals and Edo Tensei restores you to how you died.
tl;dr
Madara and Orochimaru very possibly know each other; which explains how Madara knows the seals of Edo Tensei and how he has a Cursed Seal on him.
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Look at the part where Kabuto says about being impatient may cause the opposite effect of what Itachi really wants. Then it cuts straight to Madara.
That right there was foreshadowing Madara releasing Edo Tensei, and Kabuto KNEW that was going to happen. Wait, how?
You must be registered for see links
We knew Madara lived until Nagato was young, he likely died around that time as well. Orochimaru probably just started to research forbidden jutsu and probably learned Edo Tensei then. So I'm fairly sure Madara and Orochimaru have met, and that is how Madara learned of Edo Tensei's seals.
Wait, didn't Madara learn it from Tobirama, the creator of the jutsu? I'm sure Tobirama has almost NEVER used this jutsu, maybe once to test it out. It requires a human sacrifice, and no one would allow a Hokage's brother/Hokage to use human sacrifices. Secondly, Tobirama or his brother made the jutsu forbidden for a reason, one shouldn't use forbidden jutsu, that's why its well... forbidden.
Another subtle hint:
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Why in the WORLD would Kabuto call himself Tobi's assistant? Now, I maybe just speculating here, but does Kabuto have any allegiance to anyone besides Orochimaru? He might've very well known that Madara knows Orochimaru in some way, and he is the one who uses Edo Tensei, so Kabuto simply said I am his assistant.
You must be registered for see links
This is complete speculation, but Madara is holding the back of his neck after a panel zoomed on Anko's neck, showing her Cursed Seal. That could very much be implying that Madara might have a Cursed Seal, this could mean only one thing: Orochimaru gave him it. Only Orochimaru has been shown to give Cursed Seals and Edo Tensei restores you to how you died.
tl;dr
Madara and Orochimaru very possibly know each other; which explains how Madara knows the seals of Edo Tensei and how he has a Cursed Seal on him.