HE SAID WHAT
watches the clip
...
HE SAID WHAT
Burdened by knowledge (watched anniversary streams) <-
The color on hero_sword's hair and mantle!tenna's sprites are also drastically different from the non-mantle dark world designs of the characters they represent. That means very little, imo.
Would you believe me if I said this exact topic came up in the same anniversary as 'cool mantle bro' was said as a joke. 
FU(N)
That's his scar. Every Spade person has Bellymouthes.
Wait-
Someone already said it
I don't see how it couldn't be that you're right (I like this theory) and that that is the shadow mantle. Could be that it got torn off before, put with seam (the other broken thing) then glued back later on. Doesn't explain why the knight deals less against it though.
What if the Mantle wasn't with Seam since Ch. 1 and was possessing King to make him angry?
In-game, it says the Dark Fountain made him a bit tyrannical, but every other Darkner in every other Chapter doesn't get affected the minimum by a Dark Fountain in vicinity, so what if ERAM was possessing King and that Mantle was his True/Non-Zelda form?
someone also said this, no?
harbinger migth be on something
onto something 
on something 
Toby calls it a mantle, so it's obviously intended to be one.
Ok so what if,
The shadow mantle is a flake of king, but is also the shadow mantle from Eram,
and king's broken dream of living in harmony with the lightners, gave it shadow crystal like properties, Making it stronger, and better at protecting from dark attacks, due to it itself being dark.
ERAM is most likely the Someone, so most likely They The M. Remember that kids.
In the 6th Anniversary stream, Toby Fox calls it a mantle after it flies away. it's a mantle.
I personally buy the fat that it's the Shadowmantle. Seam says they were gonna give it to you, meaning they had to have had it on them to sell. Taken they're a Chapter One darkner, that'd mean the mantle is from Card Kingdom.
You don't know what a mantle is, do you?
Okay actually it might be a cloak as opposed to a cape
