A field of 5 monsters can easily go into strong Synchro, Xyz, or Link plays depending on the deck, and you wont even need The Legendary Defenders instant win condition as decks with 5 monsters can easily reach a game-ending board
The Night King's Cycle I originally made a single Night King's card (Night King's Sight) but realized there were many more aspects of the Night King and ended up creating an entire cycle of cards within the Whitewalker cards dedicated to his awesomeness
Also according to the post, the bonus card for the Volume 2 Secret Lair is Killer Rabbit of Caerbannog, a re-themed version of the Tempest card Kezzerdrix
It was included in that set and the second set of that block, Dark Ascension
You need to be able to attack with it to get it to work, but having first strike helps a lot with that
Play the cards that bring you joy, not the ones which might be considered objectively best. Runner-Up: Tamiyo, the Moon Sage Black Crypt Rats, a Visions classic, found a reprint in Seventh Edition and Modern Horizons, but has been mostly missing across the years