Cryogen

Cryogen enters the battlefield with three charge counters on it.
When Cryogen would leave the battlefield, instead remove a charge counter from it and put a stun counter on target nonartifact creature.

I assumed that frigid mass was an old construct of his, running amok without its master to shepherd it.
Permafrost was an old ally of mine, wielding the prestigious title of Archmage with great renown.