Are the slam attacks supposed to be in addition to other attacks (natural?) of the base creature or to replace them?
I think we may end up needing to bump the CR of these slightly.
Why not make it "in addition to" like a normal zombie? Or most other undead, for that matter?
Well, right, but they have a choice of what attack to make. Maybe the skeleton, which also retains all natural attacks and weapon proficiencies (except which require flesh) and gain claw attacks, is a better comparison.
Well, I'm not sure if the CR table is quite right. The samples look better than their CR, I think. I think I'd push the CRs in the table up a step each to be a little more comfortable with it.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.