Carcosa, Accursed

Posses {4}{u}{b} (If you cast this card for its posses
cost, it's a legendary Enchantment spell with enchant
artifact. It becomes a creature again if it's not
attached to an artifact.)
Whenever you cast an artifact spell, draw a card.
Enchanted artifact has "Whenever this artifact
becomes tapped, you may pay {1}. If you do,
draw a card."