![Image](https://c1.scryfall.com/file/scryfall-cards/border_crop/front/b/6/b6534049-3045-4546-b1e8-e5b1b0df5f56.jpg?1622388359)
I've seen this card played twice now in Cockatrice. Fortunately, both times the card was quickly removed because players immediately realized how dangerous it is. However, this quick-thinking also foretells a possible ubiquity and brokenness of the card.
Both times the owner chose creatures, and I assume this will be the most common option in games. There is a reason for this: by choosing creatures, you're pretty much saying that all combat damage dealt to you and creatures you control is prevented, and your creatures become unblockable. At just seven mana, Serra's Emissary completely overshadows Blazing Archon and protects itself better than Platinum Emperion. It is for sure not an Iona, Shield of Emeria, since players can continue to cast spells and interact with the table in other ways. However, its floor is very high and its ceiling is over the roof if you have plenty of creatures on the field (Craterhoof Behemoth style). Clones are particularly obnoxious with it because you can keep choosing other card types and reduce interaction even further.
All in all, not ban-worthy in my opinion, but definitively a card to keep an eye on at your commander games. What does everyone else think?