Creatures you control are Slivers in addition to their other creature types.
Rath’s evincars used hivestones to control the slivers, but the overlay put their power into unwitting hands. Now the stones make not masters, but slaves.
If a creature is already a Sliver, Hivestone has no effect on it.
Turns only your creatures on the battlefield, not in other zones, into Slivers. It won’t allow you to have Root Sliver on the battlefield and make your Grizzly Bears uncounterable, for example.