@Cubic I wasn't disagreeing with you, but rather trying to explain how this could be unclear to some. Like you say, communication is a 2 way street, so trying to add perspective. The situation I was trying to describe was say creature wearing adamantine armor has a total AC of 20. An attack roll of 19 is ONLY a hit for a character with the Improved Crit ability because it grants a crit on a roll of 19, and then the adamantine armor reduces that to a normal hit. I think GcL's answer does a better job of explaining this.

@Cubic I think where this becomes unclear is if the creature with the adamantine armor has a total AC > 19. Then the 19 attack roll would not be a hit for any character, except for the fighter with Improved/Superior Crit. That said, based on the wording of both the armor, and the abilities, I agree it should become a normal hit, as neither actually states anything about AC.

@Medix2 Thanks for pointing that out, that's quite an important distinction I had totally missed. Fixed it in my answer, as well as addressed better the reaction/trigger timing issues.

It seems you've answered your own question while I was writing my answer, even quoting the same text as I did. What makes you think that the monk might not be able to use the action?

@Dezvul No worries, it can certainly be ambiguous. As a general rule of thumb, there's no flavor text in 5e spells, and for the most part you can read all of the rules in a pretty literal sense. Blindsight for example (also under the "Vision and Light" section of the PHB) specifically says a creature can "perceive it's surroundings." I believe if truesight was a 6th sense, they would have used similar wording.

I would be curious (more from a fluff than RAW perspective) why the ring can see through some materials but not others. Lead blocks the vision, is it simply the same for flesh? Or is it something about the life within? What about the frozen carcasses of cows hanging in the butcher's meat locker? What about a creature under the effect of Flesh to Stone? It's still a creature, but now made of an allowed material.

As far as I can tell, they're pretty much just standard doors. The inhabitants of the region are human. That quote came from a sidebar about "common features have these stats unless otherwise stated." Presumably so the author didn't have to provide a DC for every door throughout the text. Unfortunately, I don't have a link to the module itself, here's the product page from WotC: dnd.wizards.com/products/tabletop-games/rpg-products/…