Activate Ashnod's Altar by sacrificing Eternal Scourge, adding CC.
Eternal Scourge is exiled instead of being put into your graveyard due to Rest in Peace.
Cast Eternal Scourge from exile by paying 3.
The Thirteenth Doctor's first ability triggers, putting a +1/+1 counter on Workhorse.
Activate Workhorse by removing a +1/+1 counter from it, adding C.
Repeat.
This clever loop turns the exile zone into your personal hand, driven by Eternal Scourge's ability to be repeatedly cast from exile. While Ashnod's Altar provides the colorless mana to recast it, Rest in Peace ensures that the Scourge is cleanly redirected back to exile whenever it hits the graveyard, bypassing any permanent loss. Meanwhile, The Thirteenth Doctor watches you cast spells from outside your hand and hands out +1/+1 counters that fuel Workhorse for extra mana, while also setting up a massive end step untap via Team TARDIS.