Titania, Nature's Force + Underworld Breach + Phyrexian Altar + Healer of the Glade

Powerful

How it works

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Activate Phyrexian Altar by sacrificing Healer of the Glade, adding .
Healer of the Glade dies, triggering Titania's second ability, causing you to mill three cards.
Cast Healer of the Glade from your graveyard for its escape cost by paying and exiling three other cards from your graveyard.
Healer of the Glade enters the battlefield, causing you to gain three life.
Repeat.

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

1–2 · Exhibition & Core

Spellbook classifies this as Powerful — not for these brackets.

3 · Upgraded

Allowed in this bracket.

4–5 · Optimized & cEDH

All combos are allowed.