testamentary(a = adj.pert) - of or relating to a will or testament or bequeathed by a will or testament;
Derived form noun testament₂

1.  Of or pertaining to a will, or testament; as, letters testamentary. [1913 Webster]

2.  Bequeathed by will; given by testament. [1913 Webster]
"How many testamentary charities have been defeated by the negligence or fraud of executors!" [1913 Webster]

3.  Done, appointed by, or founded on, a testament, or will; as, a testamentary guardian of a minor, who may be appointed by the will of a father to act in that capacity until the child becomes of age. [1913 Webster]

testamentary, adj. of or by or in a will.

Etymology:
L testamentarius (as TESTAMENT)

letters testamentary, testamentary trust