It is the past perfect passive
المبني للمجهول لجملة الماضي التام

القاعدة لها هي:
Had been + past participle
التصريف الثالث للفعل + Had been

Example:

Active: Past Perfect
ماضي تام معلوم
Ahmad had written a letter.

Passive: Past Perfect
ماضي تام مجهول
The letter had been written by Ahmad


وفي جملتك تتضح القاعدة:
Before he died, he had been sentenced by someone


I hope it's clear

Sorry Azuma, I did not see your post