Mikaeus, the Unhallowed + Phyrexian Altar + Grazing Kelpie

Mikaeus, the Unhallowed

Phyrexian Altar

Grazing Kelpie

How it works

1. Activate Phyrexian Altar by sacrificing Grazing Kelpie.
2. When Grazing Kelpie dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Activate Phyrexian Altar by sacrificing Grazing Kelpie.
4. When Grazing Kelpie dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
5. Repeat.