That depends on who or what you're talking about.
Jeff was a cannibal, he had eaten his brother.
Jeff's brother had been eaten.
I don't know why I felt the need to refer to cannibalism in order to provide this answer. I am not a cannibal, just in case you're wondering.
"Had been eating" is the past perfect continuous of eat.
The present perfect continuous tense of eat is:I/You/We/They have been eating.He/She/It has been eating.
Existed is the past tense. The past perfect tense is had existed.
The past tense is bled, and the past perfect tense is had bled.
The past tense of sit is sat; the past perfect tense is had sat.
The past perfect tense of hope is had hoped.
The past perfect tense is had submitted.
The past perfect tense is had chosen.
The past perfect tense is "had confronted".
The past perfect tense is had set.
The past perfect tense is had burned.
The past perfect tense is had rung.