Succession clauses in an antenuptial contract are an exception to the rule that agreements relating to inheritance are unenforceable. The parties may make provision for the succession of the survivor of them to the estate (or part thereof) of the first dying spouse. Such a provision cannot be revoked unilaterally in a subsequent will without the consent of the benefiting party.
Examples "The parties declare that on the death of either of them the survivor shall be entitled to the whole estate of the first dying." "The parties declare that upon the death of the first dying of them, all assets belonging to such a party shall pass to the surviving spouse." The parties may also insert other testamentary provisions in the antenuptual contract, for example, revocation of previous wills, appointment of executors and heirs, establishment of a mortis causa trust etc.
It is also possible for a third person to bequeath his or her property to one or both of the spouses in the antenuptial contract. Just remember that the third person must then be joined as a party to the contract and he or she must also sign the contract in the presence of a notary. Such a bequest can not be revoked by the third party without the consent of the benefiting spouse(s).