The Sibsig Ceremony + Myr Retriever + Scrap Trawler + Heartless Summoning

The Sibsig Ceremony

Myr Retriever

Scrap Trawler

Heartless Summoning

How it works

1. Cast Myr Retriever by paying 0.
2. Myr Retriever enters and dies as a state-based action due to having 0 toughness. Myr Retriever triggers, returning Scrap Trawler from your graveyard to your hand.
3. Cast Scrap Trawler by paying 0.
4. When Scrap Trawler enters the battlefield, The Sibsig Ceremony triggers, destroying Scrap Trawler and creating a 2/2 Zombie Druid creature token.
5. When Scrap Trawler dies, it triggers, returning Myr Retriever from your graveyard to your hand.
6. Repeat.