We already run one Endurance mainboard; note we cannot add a second copy since none is in our sideboard, so we lean on extra Tormod's Crypt to exile their graveyard and disrupt any Agatha's Soul Cauldron plans. Unholy Heat gives us cheap interaction to remove key creatures like Devoted Druid and Vizier of Remedies before they can combo off. This is a fast combo mirror where both decks are racing to resolve their engine, so we keep our own mana development (Mox Amber, Mox Opal, lands) fully intact and only trim slow utility cards.