Huh. Trying to figure out how to break this by exiling cards in graveyard, but that still requires a bit of trickiness, since most cards that exile either go one at a time, or exile everything.

Ideally, you want to flicker the graveyard instead. You can split a Boneyard Parley, for example, so your opponent might set themselves up to return four creatures, or return your best creature and give the Portal four counters. But usually cards that operate like this are both expensive and require setup, so a four cost enchantment isn't a big deal there.

I will say, though, that multiple Weakened Portals feed off each other. Three Portals roughly means that I can guarantee that three creatures are nigh invincible, each creature bouncing in and out of the graveyard and charging the other Portals. Or mix and match which creatures are coming out.

But otherwise, pretty solid. I know this is for a cube so it probably doesn't matter. But it's an interesting engine card none-the-less.

Oh, also, this card works at instant speed. Probably fine, but maybe a pie bend, and worth pointing out if you didn't notice. Plus that part might be really strong depending on the deck. The ability to return a Frilled Mystic when tapped out is no joke. It's even better if you have a sacrifice outlet that doesn't require mana either.

Thanks for the comments, jmg. I always appreciate them.

I originally conceived this as a trigger, so while I realized it was instant speed now I didn't really stop to consider it. I think I'll move it back to a trigger.