Good with your W? His W already offers a good enough slow. The extra slow the item gives is worthless. It is simply a ball of stats. You would be much better off with some resistances like Dead mans, Spirit Visage and etc. that have good passives. Just buying Health alone isnt that good considering how many champs have %health damage. The AD Frozen mallet gives is also laughable.

Consider, if you will, the possibility that it works because you make it work, not because it is a good choice. I'm speaking from personal experience here, some years ago I would build Frozen Mallet on Nasus which, while it may work, it is way below optimal. Try some other items instead and see for yourself what you like, if you do like it then continue. Experiment.