What is Haeres Necessarius?

Legal Definition
In the civil law. A necessary or compulsory heir. This name was given to the heir when, being a slave, he was named "heir" in the testament, because on the death of the testator, whether he would or not, he at once became free, and was compelled to assume the heirship. Inst. 2,19, 1.
-- Black's Law Dictionary
Legal Definition
An heir who had no election but to accept.
-- Ballentine's Law Dictionary