There is no liability to Spanish succession tax on assets in an offshore trust.
The assets within the trust are free from Spanish wealth tax and Spanish income and gains tax.
Where the settlor (person who set up the trust) dies in Spain as a non-UK domicile, there is no liability to UK inheritance tax on assets held in certain trusts.
The assets within a trust pass to the heirs without any probate.
The trust enables various family situations to be dealt with fairly.
For example:

o Capital can be left to provide for a second wife (or husband) with the benefit to pass to the children of an earlier (or later) marriage. The trust avoids the problem of leaving assets outright to the surviving spouse (or indeed children of an earlier marriage). This way, both the surviving spouse and the children are protected.
o Where a spouse, or even a settlor, becomes incapacitated, the trustees can co-ordinate protection for them dealing with medical and care needs, as well as full financial support.
o Asset protection: assets in the trust are protected from any beneficiary becoming bankrupt.
o Assets in a trust are usually protected from a beneficiary suffering a divorce.
o Assets in a trust are protected from a beneficiary squandering money.

Where benefit is received from a trust by a Spanish tax resident, then tax will be payable in Spain.

Offshore trusts have been used for many years as a way of avoiding tax legally and, for individuals, are probably the single most effective tool for this purpose. Offshore trusts enjoy many valuable tax privileges, and a close study of them can reveal many ways of how they can be used to advantage. This does not apply to just the very wealthy. Trusts are complicated and professional advice must be sought.

Source: BlevinsFranks