mtg 15 card booster pack Dominaria Magic 2015 introduced a templating change to cards that set a creatures power and toughness to a specific value Magic 2015 Core Set (m15):

Whenever you cast an enchantment spell, create a white Spirit Cleric creature token with This tokens power and toughness are each equal to the number of Spirits you control. Illustrated by David Auden Nash Standard Not Legal Alchemy Not Legal Pioneer Legal Historic Legal Modern Legal Brawl Legal Legacy Legal Comp

Its a fixture in the Legacy metagame, mainly because there are so many ways to customize the deck to suit your playstyle

2 Rem Karolus, Stalwart Slayer One of the biggest problems facing go-wide Commander strategies, such as Knights, is the prevalence of board wipes in the format

They are rare collectibles, which means there are not a lot of them for sale and people who crack them, expect to have hit the jackpot