Isn't this (covenants that basically have dead people in their graves reaching out to smite living people) exactly what the dead hand rule was created to prevent? This was a major part of the "defeudalization" that took place between the 17th-19th centuries in most of western Europe, as before then the nobility would entail their estates so as to keep them whole in the senior male line. It does allow for limited postmortem control, but practically not more than one human lifespan thereafter.

C.f. https://en.m.wikipedia.org/wiki/Rule_against_perpetuities