Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight + Phyrexian Altar + Highspire Artisan

How it works

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Activate Phyrexian Altar by sacrificing Highspire Artisan.
When Highspire Artisan dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
When Highspire Artisan enters, its fabricate ability triggers, putting a +1/+1 counter on it.
Repeat.

In which brackets is this combo allowed, per Spellbook's Spicy tier?

1–2 · Exhibition & Core

Spellbook classifies this as Spicy — not for these brackets.

3 · Upgraded

Allowed in this bracket.

4–5 · Optimized & cEDH

All combos are allowed.