Agatha's Soul Cauldron + Wildwood Mentor + Phyrexian Altar + Pentavus

Agatha's Soul Cauldron

Wildwood Mentor

Phyrexian Altar

Pentavus

How it works

1. Activate Wildwood Mentor using Pentavus's first ability by paying 1 and removing a +1/+1 counter from it, creating a 1/1 Pentavite artifact creature token.
2. When the Pentavite enters the battlefield, Wildwood Mentor's first ability triggers, putting a +1/+1 counter on itself.
3. Activate Phyrexian Altar by sacrificing the Pentavite, adding 1.
4. Repeat.

This loop relies on Agatha's Soul Cauldron exiling Pentavus, granting its activated abilities to Wildwood Mentor and Phyrexian Altar once they have +1/+1 counters. You can then spend the mana generated by sacrificing the generated token to Phyrexian Altar to pay for Pentavus's counter-removal ability, which creates a Pentavite token to fuel the whole cycle again. Every time that token enters the battlefield, Wildwood Mentor triggers to stack another +1/+1 counter on itself, keeping your engine's power running smoothly.