The situation depends if you and your heirs are resident or non resident in Spain (in this case, in Valencia region).
Residents in Valencia are currently exempt of payment of inheritance tax.
Non residents pay the inheritance tax to the central government regardless the location of the property.
The criteria to calculate the inheritance tax are various: the value of the asset, the relationship between heirs and the previous owner, the value of the assets owned by the heirs prior to accept the inheritance, etc.
There are various measures that can be taken to reduce the impact of the inheritance tax, but my advice is that you discuss your case with a professional legal advisor in a proper manner. The information you can get through forums could be worthy in general terms but at the end of the day you have to take your decisions based on accurate advice focused in your particular case.
MT Legal Abogados