This and Droning Philosopher are the examples for common law cards.

Is the Praetorian Guard a Roman-specific thing? It isn't now.

It was, but sure. Though this kinda conjures up a world where "Praetorian" means "Kitty".

Law is an interesting mechanic; I do wonder if the deck it builds won't spiral out of control, though.

I will definitely change the name; I just wanted something cool and law sounding for the concept.

It definitely is something I am keeping an eye on. I am trying not to have too many clauses with potential for overlap - gonna focus on noncreature types for now, as well as timing. I figure CMC 5 or greater is mostly fine since few common noncreature spells cost 5 or more. I am tempted to include "targets a creature you control" as a law but doubling up is likely with the law creatures that block instants or sorceries. This is definitely a mechanic that will need some extensive testing to find what is just the right amount of oppression.

I think my other concern with this mechanic is that it, Oath, and Battalion both lend itself to linear decks that want to snap up as many similar cards if possible, leading to on-rails drafting in White (though maybe that might be somewhat of a good thing to encourage less fighting for cards within the color?)