Cavern of Souls is another land that can be dangerous if your interaction suite is heavy on counterspells
Sapphire continues the trend from Rise Of The Floodborn of showing how powerful synergy between characters can be with Huey, Dewy, and Louie, particularly with Huey's ability, Three Nephews, which allows you to draw three cards if one of each of the Nephews is on the field
And summon the Avatars, big creatures that fight on behalf of their respective color
(See rule 509, Declare Blockers Step.) 702.111c Multiple instances of menace on the same creature are redundant
Nadu is easily one of the most impactful cards to be printed in years
No, for the cards that ask you to choose a creature type, you must choose a type that exists in Magic