It depends on how powerful it is.
Honestly, it would be better for shields to be brought into line as opposed to a hard counter for shields to be introduced.
If one were introduced, it probably would be unhealthy, since it would either be an always build or a never build based on the meta. In HotS, Varian has the talent choice of Shattering Throw so he can opt into it to counter enemies with shields; this balances out because HotS has no items, and thus not every character has shields (No Locket, for example). Meanwhile his other choices on that tier can grant him other utility like %-health damage to bust tanks or reduced healing. The fact that HotS has no items actually keeps that talent balanced.
In LoL, every character technically has access to shields through Locket, for example, or Masteries like CoC, on top of champion abilities. It wouldn't be a situational buy, then, as much as it would be a MUST buy against teams with a Locket - which is almost every game.
It would make a neat way to rework
.
+
+ 600g - 3100g total
30 AD
10% Life Steal
400 Health
Unique - Active: Throw the Frozen Mallet at the target enemy, dealing 200 Magic Damage (Increased by 500% VS Shields) and slowing the target by 25% for 2 seconds.
Just tinkering. Probably not very balanced.
Yeah, it would be better to just nerf shields.