Preston, the Vanisher + Ashnod's Altar + Karmic Guide + Chord of Calling

Decks

How it works

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Cast Chord of Calling by paying at most , searching your library for Karmic Guide and putting it onto the battlefield.
When Karmic Guide enters, it and Preston trigger.
Resolve the Karmic Guide trigger, returning an applicable target from your graveyard to the battlefield.
Holding priority, activate Ashnod's Altar by sacrificing Karmic Guide.
Resolve the Preston trigger, creating a token copy of Karmic Guide that is a 0/1 Illusion.
When the Karmic Guide token enters, it triggers, returning Karmic Guide from your graveyard to the battlefield.
Repeat from step 2.

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

1–2 · Exhibition & Core

Allowed in this bracket.

3 · Upgraded

Allowed in this bracket.

4–5 · Optimized & cEDH

All combos are allowed.