At the beginning of your combat phase, Ayara triggers, returning Scourge of the Throne from your graveyard to the battlefield, and giving it haste until end of turn.
Declare Scourge of the Throne as an attacker, attacking an opponent whose life total is higher than or equal to yours.
Scourge of the Throne's dethrone ability and second abilities trigger.
Resolve the second Scourge of the Throne trigger, causing you to untap all attacking creatures and to get an additional combat phase after this one.
Holding priority, activate Ashnod's Altar by sacrificing Scourge of the Throne, adding CC.
Resolve the second Scourge of the Throne trigger from step 3, doing nothing.
Repeat each combat phase.
This brilliant loop hinges on Ayara, Widow of the Realm // Ayara, Furnace Queen's combat-step trigger, which continuously resurrects Scourge of the Throne from your graveyard with haste. When Scourge of the Throne attacks the player with the most life, its dethrone ability and extra-combat trigger stack up, allowing you to sacrifice it to Ashnod's Altar for colorless mana just before the extra combat phase grants you another chance to repeat the whole process.