Reducing a creatures toughness to 0 means its put into the graveyard the next time state-based actions are checked, usually before a player gains priority
The Unfathomable Crushing Brutality of Basic Lands - Black and white, metal (the style, not the material) versions of the five Basic Land cards
If you are using it defensively to protect your own spells on your turn, I would cast it in your second main phase, especially if you do not care about attacking
But for other Rooms where both abilities are one-use, or more context-dependent effects, there will likely be a dynamic choice about which order to unlock the doors every time theyre played
Poison Dart Frog is a real cutie
Consequently, the fact that Death's Majesty makes all Zombie creatures is very good since her ultimate becomes a one-sided board wipe that will likely win you the game on the spot