Soulflayer

• WOTC Official Reminder Textofficial rules
  Delve — Each card you exile from your graveyard while casting this spell pays for .

• WWizards of the Coast2014-11-24
  Official Ruling
  Creature cards that have abilities that grant the listed keyword abilities to themselves won’t count. For example, exiling a Battle Brawler (a creature with “As long as you control a red or white permanent, Battle Brawler gets +1/+0 and has first strike”) with Soulflayer’s delve ability won’t cause Soulflayer to have first strike, even if you control a red or white permanent.
• WWizards of the Coast2014-11-24
  Official Ruling
  You can’t exile more cards from your graveyard than you’d need to pay Soulflayer’s generic mana requirement. In most situations, this means you can’t exile more than four cards, even if you want to exile more to give it extra abilities.
• WWizards of the Coast2020-01-24
  Official Ruling
  If one of those creature cards has one or more variants of the listed keywords (for example, hexproof from white), this creature gains those specific variants.
• WWizards of the Coast2021-03-19
  Official Ruling
  Delve doesn’t change a spell’s mana cost or mana value. For example, Treasure Cruise’s mana value is 8 even if you exiled three cards to cast it.
• WWizards of the Coast2021-03-19
  Official Ruling
  You can exile cards to pay only for generic mana, and you can’t exile more cards than the generic mana requirement of a spell with delve. For example, you can’t exile more than seven cards from your graveyard to cast Treasure Cruise unless an effect has increased its cost.
• WWizards of the Coast2021-03-19
  Official Ruling
  Because delve isn’t an alternative cost, it can be used in conjunction with alternative costs, such as flashback. It can also be used to pay for additional costs that include generic mana.

Rulings via Scryfall