Destroy all creatures, then create an X/X colorless Phyrexian Horror artifact creature token, where X is the number of creatures destroyed this way.
If a creature regenerates or has indestructible, it won't be counted when determining the value of X.
Any triggered abilities that trigger because a creature was destroyed won't be put onto the stack until after Phyrexian Rebirth finishes resolving. This means the Horror creature token can be targeted by those abilities, if applicable.