Karn, Silver Golem + Erratic Portal + Ugin's Nexus + Ultron, Artificial Malevolence

Decks

How it works

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Cast Ugin's Nexus by paying .
When Ugin's Nexus enters, Ultron triggers, causing you to pay to create a token copy of Ugin's Nexus.
Put the token Ugin's Nexus into exile due to the legend rule and its replacement effect, causing you to take an extra turn after this one.
Activate Karn by paying , causing Ugin's Nexus to become a creature until end of turn.
Activate Erratic Portal by paying and tapping it, choosing not to pay and return Ugin's Nexus from the battlefield to your hand.
Repeat each turn.

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

1–2 · Exhibition & Core

Spellbook classifies this as Ruthless — not for these brackets.

3 · Upgraded

Spellbook classifies this as Ruthless — not for these brackets.

4–5 · Optimized & cEDH

All combos are allowed.