I say "again" because I did a search first and found a lot of ideas provided by Stamp and others. I am not trying to "steal" the Cyrus idea, but rather consolidate it to what we know from the Scriptures, to make his ability actually useful, and to make his ability easy to understand.
King Cyrus
8/12 White/Purple Hero, Warrior-Class Persian King (warrior class is questionable, so I'm going off some secular history here)
When this hero enters play, immediately return all captured OT heroes to owners' territories. No OT hero can be captured as long as Cyrus remains in play. Cannot be negated.
Ezra 1:2-3 "Thus says Cyrus king of Persia, 'The Lord, the God of heaven, has given me all the kingdoms of the earth, and He has appointed me to build Him a house in Jerusalem, which is in Judah. Whoever there is among you of all His people, may his God be with him! Let him go up to Jerusalem which is in Judah, and rebuild the house of the Lord, the God of Israel; He is the God who is in Jerusalem."