Not only does it halve their attack, but it also requires the owner to spend quanta to unburrow them. When you have an element that also utilizes quanta denial such as Earthquake, this can become a powerful stalling tool. While I do like the card, I can't help but wonder if it doesn't cost just a little too much? Personally, 4
/3
feels a little bit better for this, and possibly even lower since it affects all creatures. Now that I think about it, you could give this card a similar dynamic to Pandemonium and make it affect all creatures unupped for a lower casting cost, whereas the upped version could have a greater cost and affect only your opponent's creatures.