As far as Darkness-type cards go, Nidoking has the most HP of them all and a consistent way to deal damage each turn
While not mentioned in the FAQ, another reason is that placing tabs will add extra thickness to the surface, and if you had tabs attached to roughly the same spot on all sleeves, that thickness will add up to form reliefs and damage the cards if the band you use to hold the cards was too tight
SE Secret Rare Magical Contract Door X SPELL Add 1 Spell Card from your hand to your opponent's hand, then add 1 Level 7 or 8 DARK monster from your Deck to your hand
Creature, land, artifact, you name it, Vampiric Tutor can get it, and get a win condition, board wipe, or anything else you might need given the current game state
cookies.
Lower Text [All Turns] [Once Per Turn] When this Digimon deletes your opponent's Digimon in battle, trash their top security card