Alright, for those of you who are not aware, I found a much simpler way to do this.
There is a creep ability "parasite". Copy that, change the damage per tick to 4, change the movement speed to -0.75 (shift+double click to set a negative value), set the allowed targets to ground, air, and organic, and reduce the number of voidwalker summons to 0. Then, of course, change all the icon/tooltips/mana cost/etc to match unholy frenzy. This is already an auto-cast ability, so it's as simple as that.
Note: It was not a typo to adjust the movement speed. I guess it's a bug, but movement speed affects attack speed and attack speed affects movements speed..