I like the attempt to make an interesting weapon less useless. However...The crack effect seems way OP.
It's easier than breaking concentration through normal damage with a high damage roll of ~20 hit points (which also requires a roll to hit). It's easier than counterspelling which A) requires you know the spell and have it prepared, B) requires you spend a spell slot, C) could require a roll on your part if the spell is higher than your spent slot, and D) requires your reaction.
This feat gives you an unlimited free counterspell you could use every single turn with the only real limitation being range (a 15ft radius is huge and covers 36 squares beside the one you occupy), and your reaction (which is required for counterspell anyway). What's more is it puts the roll on the caster instead of the attacker with success weighed against them. It's basically the prefect anti-mage feat. With a whip of all things.
Maybe if it was more limited, it would be less over powered but still useful. Making it only usable a few times a day wouldn't really make sense flavor-wise. But it could require a reasonable skill check to reflect their skill with handling a unique weapon and only work one time on the same caster in a single encounter as they should be expecting it next time.