To spot fake cards, look for font irregularities, missing accent marks on the word Pokmon, a flat or purely vertical rainbow sheen instead of diagonal foil patterns, a lack of physical fingerprint-style surface texture on modern ultra-rares, and the absence of the dark middle cardstock layer visible on a genuine card's edge
As such, a player does not need any other expansions to play them or encounter cards from the core boxonly the player cards
It gives a huge advantage with mulligans, especially with the new London Mulligan First hand, you've seen 7 cards + 5 OUAT Second hand, you've seen 14 cards + 5 OUAT Third hand, you've seen 21 cards + 5 OUAT Once Upon a Time serves as the missing link in whatever you need during mulligans
Once per turn, when a "Dark Elf" monster you control declares an attack, your opponent loses 1000 LP