noun
testator
Legal term for a person who makes a will.
遺言者は、遺言書に署名しなければならない。
The testator must sign the will.
被相続人 is the decedent (the person whose estate is being inherited), which is often the same person as the testator, but the terms are used in different legal contexts.
Compound of 遺言 (will, testament) and 者 (person).