How about a dark zap spell like the one in Clash Royale?

The zap spell can be created from dark spell factory, with low damage (maybe only dead to skeletons) and small range, but some awesome side-effects, just like the one in Clash Royale:

1) causing defensive building / cc troops to re-lock targets,
2) causing inferno tower to re-stack damage.
3) creating very short (0.5-1 second???) damage break of defensive buildings and stun of cc troops.

- What makes this unit/spell unique compared to the other units/spells in the game
Defensive building / cc troop damage break and function reset.

- What kind of strategies this new unit would enable (concrete examples are best)
An alternative to earth quake and freeze spell, especially towards single target inferno towers (re-set), but it cannot replace freeze spell due to very short damage break. More importantly, it can help save balloons in "Lava hound+ balloon" strategies, in case an air defense / archer tower locks a balloon first, due to lava hounds deployed slightly late.

- What kind of army composition includes this unit or, how it could combine with other units for more advanced strategies
Besides the advantage introduced in the last answer, it is expected that the one poison + one zap combination will be more popular than 2 poisons in hogs riding, since it is dead (only) to skeletons.

- How someone could redo their base to protect against attacks from this unit
Disperse defensive buildings can mitigate the effect of zapping, since the spell only has a small range of attack.