You have not cast a spell using Demon of Fate's Design's ability this turn.
You control at least seven nontoken creatures.
Steps
Cast Day of the Dragons by paying 7 life due to Demon of Fate's Design.
When Day of the Dragons enters the battlefield, its first ability and Cloudstone Curio trigger.
Resolve the Cloudstone Curio trigger, returning nothing to your hand from the battlefield.
Resolve the Day of the Dragons trigger, exiling all creatures you control and creating that many 5/5 Dragon creature tokens.
When the Dragon(s) enter the battlefield, Cloudstone Curio triggers that many times.
Resolve all but one Cloudstone Curio trigger, returning nothing to your hand from the battlefield.
Resolve the final Cloudstone Curio trigger, returning Day of the Dragons to your hand from the battlefield.
When Day of the Dragons leaves the battlefield, its second ability triggers, causing you to sacrifice all Dragons you control and return all cards exiled with Day of the Dragons to the battlefield.
When the creatures that were exiled by Day of the Dragons enter the battlefield, Cloudstone Curio and Auriok Champion trigger that many times.
Resolve all Auriok Champion triggers, causing you to gain life equal to the number of creatures that entered the battlefield.
Resolve all Cloudstone Curio triggers, returning nothing to your hand from the battlefield.
Repeat.
If you control eight or more nontoken creatures, this combo nets infinite life.