Azathoth the Abyssal Idiot said:
Not sure. Ugin gave some kind of analogy using a lake (the multiverse), a man standing above it (the true Eldrazi), and the man's hand (the Eldrazi's shadows). They effectively drove a stake through the man's hand, which stopped him from just getting up and leaving, but didn't directly affect the man on the surface.
Partly my issue as well but it is worth nothing that it is likely the laylines of Zendikar that sealed the Eldrazi and their shadows And they never terrorized the multiverse again until Chandra and Jace came along and broke them from their prison.
Their true forms are High 2-A, and their shadows are at least 2-A for being immensely above most planes-walkers, but their shadows were likely not sealed on the plane of Zendikar.
I hate this argument, but for all intensive purposes they never threatened the multiverse again, if it was just their shadows sealed, they could simply just went on else where in the multiverse, especially since they can travel through the Blind Eternites like it is no issue. If their hands were sealed, this is a moot point but I think it is more of their bodies being sealed, otherwise they could have left....maybe... probably?
I am fine with Possibly High 2-A with Prep. It is not quite enough evidence to put them concretely that tier, but I do think there is an argument to favor at least this.
Or at least
Higher with prep.
Basically pick one, but even then iirc Zendikar was a special case, no normal plane in the mtg multiverse could have sealed the Eldrazi as well as Zendikar did considering how strong the mana was for the plane, so not sure if this feat is replicable in normal circumstances.
Edit: Also, considering Ugin was the leader of the 3 and he was the one who devised the plan and is aware of the Eldrazi's existence and explained it to Jace, it does provide some credence to him being able to seal their true forms. If he knew absolutely nothing about them, sure it could just be their shadows But we know the Eldrazi are higher dimensional because of Ugin, I doubt he would have gathered two planeswalker with conflicting ideologies and just seal their shadows. Not saying he could or cant, but Ugin being aware of their existence and how they does lead to me to be inclined to believe he developed the plan with this in mind, and not their shadows. Although this could be a reach.