Incinerator of the Guilty

Flying, trample
Whenever this creature deals combat damage to a player, you may collect evidence X. When you do, this creature deals X damage to each creature and planeswalker that player controls. (To collect evidence X, exile cards with total mana value X or greater from your graveyard.)

Murders at Karlov Manor

Flying, trample
Whenever Incinerator of the Guilty deals combat damage to a player, you may collect evidence X. When you do, Incinerator of the Guilty deals X damage to each creature and each planeswalker that player controls. (To collect evidence X, exile cards with total mana value X or greater from your graveyard.)

Flying, trample
Whenever Incinerator of the Guilty deals combat damage to a player, you may collect evidence X. When you do, Incinerator of the Guilty deals X damage to each creature and each planeswalker that player controls.

Murders at Karlov Manor Promos

If you can't exile enough cards to meet or exceed the required mana value, you can't choose to collect evidence at all.

Once you've announced that you're casting a spell, players can't take actions until you've finished doing so. Notably, opponents can't try to remove cards from your graveyard to stop you from collecting evidence.

You may collect evidence 0, but you would be a lazy detective with no crispy culprits. Nevertheless, if you choose to do so, abilities that trigger "whenever you collect evidence", such as those of Surveillance Monitor and Evidence Examiner, will still trigger.