From the YT comments, someone quoted a interview with Miyazaki. Idk if it's real or not.
"First of all, the setting of Shadow of the Erdtree is a brand new land. It's a brand new map separate from the Lands Between. It is a land that is overshadowed by the particular Shadow of the Erdtree as opposed to the Erdtree in the Lands Between. And it takes place, again, on an entirely separate, physically separate map. So it will involve a warp of sorts to get there.
In terms of setting and themes, it technically occupies the same space as the Lands Between, the same universe. But due to something story related that we won't reveal today, this has become physically disconnected, and you'll travel to the Shadow of the Erdtree land as a separate place.
[...] if you think in terms of scale or size, it's probably comparable, if not larger, than the area of Limgrave from the base game.
The cocoon and arm you saw at the beginning of the trailer this is actually the point of entry to the Land of Shadow where the players will enter to the DLC area. And this does have some relation to Miquella. And Miquella has, as we said, traveled to the Land of Shadow. He does have some motive and some objective there, which we don't want to give away too much. But essentially the player will be following in those footsteps in the same way that they followed the blessings from the Grace, the Sites of Grace in the Lands Between. They'll be following in Miquella's footsteps, and these will guide them through the Land of Shadow and reveal that motivation to them.
[...] We believe he will be officially revealed to be Messmer by the time the trailer goes live. And yes, he's a key figure in the DLC and he again has this important element of shadow to him, which is a key theme throughout the DLC as well.
And you may have seen at the end of the trailer, there was this piece of key art where it shows Messmer sat in this throne-like chair, and people who've played the game may recognize this throne to be one of those from the boss room where you battle Morgott. And this represents the thrones at the base of the Erdtree. And it's supposed to symbolize that Messmer stands on equal footing to these other demigods and children of Marika who sat around in these thrones and held the rooms of the Erdtree.
So he is an important figure who rivals these other demigods. And as you play the DLC, you will learn a little about why he wasn't featured in the legends of the Erdtree, the lands between. You'll realize why he exists in this shadow, this land of shadow."
Miyazaki IGN Interview
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Look, I can name a few instances in MY life where I tried to reach mutual understanding and i can TELL you, always faster and easier to just kill em. Just is!