Aurora Eidolon

Commander is where Aurora Eidolon earns its slot, specifically in five-color or heavy-multicolor piles that routinely cast spells with six or more colored pips — every activation is effectively a free mana that scales with spell density. In Pauper it's legal but irrelevant; the format moves too fast for a three-mana do-nothing body. Legacy and Vintage have access to it but would never run it — the effect doesn't approach the power threshold of those formats. Oathbreaker mirrors Commander's logic at a smaller table, so the same multicolor-heavy builds that want it there want it here.