­Will-o'-the-Wisp + Spiritual Conclave

Argh. The original air-stall card. So much annoyance on such a tiny thing. And a ghostly prison that you can cast for free if you're playing tribal.

Well, guess we're messing up the opponents attack, then. Maybe with a bit of tribal (and huh, I forgot 'spirit' goes all the way back to alpha; Kamigawa kinda ruined it for everyone).

Only creatures that share a tribe may attack? Only creatures that do NOT share a tribe (and then you drop a mistform for the combo)?

Pay (and maybe tap something) to block an attacking creature? That.. kinda works. And is cleaner than mass-regeneration.

Obvious name mash suggests a terirble accent. So there's the flavour.

Cost seems about ghostly-prison to me, since you have to pay to use it. And probably wanrts to be rare because seriously, you don't want a lot of these popping up in limited.

It's a damn good answer to "Oh, that attacker scares me." But it's still beatable - run more creatures, directly kill their creatures, have trample, disenchant.

Quite similar to Glare of Subdual. That needed to tap down creatures preemptively rather than waiting to see what attacks, but this one has a mana cost on the activation which is much more sensible. Looks quite printable, though it's encouraging a playstyle people often get frustrated by so you don't want many cards like this around.

While it does encourage a stall - you need creatures to do it. Soit's also sneakily encouraging you to play a combat heavy deck. The very best deck for this would probably be angels - lots of vigilance and evasion, and also available for perfect defence.