The Pokémon TCG: Scarlet & Violet—Twilight Masquerade 3-pack blister contains three booster packs from the Twilight Masquerade expansion, a special foil promo card (often a Revavroom), and a code card for the online game. The expansion itself introduces the land of Kitakami and its masked Legendary Pokémon, Ogerpon, alongside new Pokémon like Bloodmoon Ursaluna ex and Sinistcha ex.  
What's inside the 3-pack blister
3 Booster Packs: Each contains 10 cards and 1 Basic Energy from the Twilight Masquerade set. 
1 Special Promo Card: A foil promo card, most commonly the Revavroom promo. 
1 Code Card: This card can be used in the Pokémon TCG Live online game. 
Possibly a Pokemon Coin: Some versions of the blister may include a Pokémon coin. 
About the Twilight Masquerade expansion
Setting: 
The expansion is themed around the land of Kitakami, where humans and Pokémon coexist in harmony. 
New Pokémon: 
It features the Legendary Pokémon Ogerpon in four fearsome forms as Tera Pokémon ex, as well as other new Pokémon like Bloodmoon Ursaluna ex and Sinistcha ex. 
New Cards: 
The set includes more than 160 cards in total, with over a dozen new Pokémon ex and new ACE SPEC cards. 
Features: 
Look for special illustrations and cards featuring Greninja, Dragapult, and Magcargo as Tera Pokémon ex.