­Merseine

Target creature has to hunt for primes, when it find some, it can untap?

This is a really bad blue Paralyze.

Let's at least try to make this useful on your own creatures as a costly untapper. And make it significantly cheaper.

Still very much not common. And man this set wants your creatures to stall out so that you can win with, um, this set also lacks direct damage. ARGLE.

Make uncommon.

I wouldn't be surprised if Thorn Thallid is the MVP for some decks :)

(As for primes: why do you think the Merseine Sieve is so called?)

You may be interested to note that, while Claustrophobia and Paralyzing Grasp are the baseline current versions of this effect, Khans of Tarkir brings a cheaper take on it that's a bit like Merseine but still common: Singing Bell Strike.

You mean it's not because you filter the numbers through a school of mermaids?

Huh, didn't know the effect was going common. It seems a bit nastier than blue usually gets.

It's basically Dark Banishing in blue. Every colour needs common removal (well, green gets Savage Punch). Blue gets bounce, counter, and lockdown (either "doesn't untap" like the above examples, or "abilities can't be activated", or both on the same card like Encrust).

But actual common removal costs like 5+ nowadays.