had completed.
The past perfect tense of complete is had completed.
"They have" is the subject and auxiliary verb parts of the present perfect tense. Following these should be a past participle of a verb to complete the tense.
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 set.
The past perfect tense is had submitted.
The past perfect tense is "had confronted".
The past perfect tense is had rung.
The past perfect tense is had burned.
The past perfect tense is had chosen.