See Inscrutable Spellforme.

Not a big fan of this implementation. I'd prefer this mechanic to be only on permanents, so both players get to experience it in its locked state and its unlocked state. The mechanic invokes a feeling of puzzleboxes - best to keep it on cards that can invoke that sort of feeling.

I like the concept, but I have to agree with Mal, that the spell seems better on permanents. Does this become unlocked before any Destructive Spellformes are cast as long as you've cast a spell at some point? That just seems weird for a paper card game (though I could see it in a purely digital card game). Maybe "If you've cast another spell this turn, ~ is unlocked." since I think replacement sounds better here. That does force the player to have played something the same turn, like how Crush of Tentacles checks to see if the Surge cost was paid rather than just having cast a spell once. Having casted a spell once at any point just seems too easy a way to have spells unlocked.

You are right. The mechanic should stay on artifacts and lands, as I originally intended.

The unlock trigger is intended to happen while these are on the stack, which is confusing enough that I'm not sure either of you got it.

If you do want to revisit this on spells, you'd probably want to go with "When you cast another spell while ~ is on the stack, unlock ~."