TBN isn't a bad skill, it's because it's DPS neutral when it breaks and a DPS loss when it doesn't break, while every other tank has a short cooldown defense buff the DRK is the only one when used NEEDS 3000 MP aka a resource needed for part of the DRK's DPS output. Now if TBN didn't cost 3000 MP and didn't grant Dark Arts I think this would be a good compromise. However the 25% extra HP shield it gives would probably need to be nerfed slightly as well if the 3000 MP condition was removed.