Mikaeus, the Unhallowed + Bloodied Ghost + Phyrexian Altar

Mikaeus, the Unhallowed

Bloodied Ghost

Phyrexian Altar

How it works

1. Activate Phyrexian Altar by sacrifiicing Bloodied Ghost.
2. When Bloodied Ghost dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it. When it enters, Bloodied Ghost will put a -1/-1 counter on itself, removing the +1/+1 counter.
3. Repeat.