Translating legal document

[Pura]

Hi, I'm reading a pre-nuptial agreement in Spanish and there is a specific section I don't understand. Can anyone help me?
It reads : Asi lo dicen y otorgan ante mi, el Notario, despues de haberle advertido del derecho que tenian de hacerlo por si y al que renunciaron."
I understand most of it, but I think it's the latter part of this sentence that is confusing me. Thankyou !