Hogaak, Arisen Necropolis

Text:

You can't spend mana to cast this spell.
Convoke, delve (Each creature you tap while casting this spell pays for or one mana of that creature's color. Each card you exile from your graveyard pays for .)
You may cast Hogaak, Arisen Necropolis from your graveyard.
Trample

Rulings:

• 6/14/2019 You can tap an untapped creature you haven’t controlled continuously since the beginning of your most recent turn to convoke a spell.
• 6/14/2019 You can exile cards to pay only for generic mana of the spell with delve. This means that normally you must tap at least two black and/or green creatures to convoke Hogaak in order to cast it.
• 6/14/2019 When calculating a spell’s total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke and delve apply after the total cost is calculated. Convoke and delve don’t change a spell’s mana cost or converted mana cost.