DelveDelve is a static ability that functions while the spell with delve is on the stack. “Delve” means “For each generic mana in this spell’s total cost, you may exile a card from your graveyard rather than pay that mana.”
The delve ability isn’t an additional or alternative cost and applies only after the total cost of the spell with delve is determined.
Multiple instances of delve on the same spell are redundant. (CR 702.66) (Each card you
exileTo exile an object, move it to the exile zone from wherever it is. See rule 406, “Exile.” (CR 701.13) from your graveyard while casting this spell pays for
.)
Target opponent mills nine cards, then you may
castTo cast a spell is to take it from the zone it’s in (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. A player may cast a spell if they have priority. See rule 601, “Casting Spells.”
To cast a card is to cast it as a spell. (CR 701.5) an instant or sorcery spell from that player's graveyard without paying its mana cost. If that spell would be put into a graveyard,
exileTo exile an object, move it to the exile zone from wherever it is. See rule 406, “Exile.” (CR 701.13) it instead.