Spells with delve are very powerful, but usually have a high mana cost. Fortunately, you don't need to pay all of it with mana. The Sultai's ruthlessness leads them to use every resource at their disposal, even the dead. As you're paying for the spell, you can exile cards from your graveyard. Each card you exile this way pays for one generic mana.

Exploit is a keyword ability that allows you to sacrifice another creature when the creature with exploit enters the battlefield. Creatures with exploit also have a second triggered ability that occurs when they "exploit a creature."

Elspeth may choose to defy the gods of Theros, but you can instead show your devotion to them and reap their rewards. '"Devotion" is a rules term that counts colored mana symbols in the mana costs of permanents you control to determine the magnitude of their effects.