Karmic Guide + Nim Deathmantle + Ashnod's Altar + any other nontoken creature = infinite sacrifices and ETBs
How it works:
- Sacrifice Karmic Guide and another nontoken creature to Ashnod's Altar, making 4 colorless mana and triggering Nim Deathmantle's ability
- Pay 4 to return Karmic Guide to the battlefield attached to Nim Deathmantle using Nim Deathmantle's triggered ability, triggering Karmic Guide's enters the battlefield ability
- Return any creature from your graveyard to the battlefield with Karmic Guide's ability
- Repeat