changeling mtg cards War-Spike [Modern Masters] Changeling (This card is every creature type.) When this creature dies MtG Lorwyn Uncommon Foil Changeling

At that point, youre talking about an entire strategy revolving around infinite resourcessomething that should already fall under the umbrella of a Rule 0 discussion

But Cavern of Souls is the only one that has the clause that makes your creature spell uncounterable

It has some built-in help by gaining life whenever you cast a white spell, but Im thinking in bigger terms

Don't see this being a thing in any sort of cube setting though

It shows how little the designers understood the game they had created that this card - which lets you draw three cards for one mana - was assumed to be roughly as powerful as Giant Growth, Lightning Bolt, Dark Ritual, or (of all things) Healing Salve