Currently we have 3 troops that do something when they take skull damage:
Bone Dragon freezes whoever hits him
Sheggra burns whoever hits her
Hydra does damage to the team when someone hits it
We need more of these!
A legendary plant creature that snares when hit.
A mucus golem that inflicts poison and/or disease when hit.
A giant iron golem that inflicts stun when hit (consider the recoil when hitting iron with a hammer)
A giant bell that inflicts silence when hit (a stretch, but the sound makes it so nobody can hear you after)