Frostwolf Skull on defiler is an interesting item because it gives her an excellent increase in stats and hp, which she really needs, and its orb effect synergizes so well with her ulti. Her ulti does alot of dmg if focused on an enemy hero but most of the time enemy heroes run away from it with ease and chasing them wont result in effectively damaging them as the spirits are so slow to move. Frostwolf skull causes a 30% constant movement slow to the enemy hero being focused. That way you can hit them, get close to them and then just focus them and let your ulti own them. The only problem with frostwolf is that it's really expensive and you don't get it until late game, at which point chasing individual enemy heroes is dangerous. But if you are chasing with team mates you are sure that the person you are chasing won't escape alive. If you are good at lane farming / last hitting you might manage to get it relatively early. The earlier you get this item the more devastating it is imo.