Mikaeus, the Unhallowed + Phyrexian Altar + River Kelpie

Mikaeus, the Unhallowed

Phyrexian Altar

River Kelpie

How it works

1. Activate Phyrexian Altar by sacrificing River Kelpie.
2. When River Kelpie dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. When River Kelpie enters, its first ability triggers, causing you to draw a card.
4. Activate Phyrexian Altar by sacrificing River Kelpie.
5. When River Kelpie dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
6. When River Kelpie enters, its first ability triggers, causing you to draw a card.
7. Repeat.

This engine turns River Kelpie into an immortal card-drawing machine by playing off the neat mathematical interplay between its native persist ability and the undying granted by Mikaeus, the Unhallowed. Each time you sacrifice the Kelpie to Phyrexian Altar for mana, it alternates returning with a -1/-1 counter or a +1/+1 counter, effectively wiping away the previous counter type so it can keep dying indefinitely. Meanwhile, River Kelpie's own trigger rewards every single one of those graveyard entries by drawing you a card, letting you easily cycle through your entire deck while generating an endless supply of colored mana.