This is from the .com screenshots thread. After being questioned, the poster responded with:

"you get a roman eagle, when you are given a 'first cohort' by the senate, these cohorts are bigger harder and faster than the usual, they give morale boosters to all nearby units. all in all, they're pretty hardcore.

when you KILL a first cohort to a man, you can get the eagle. i'm not entirely sure what it does for you, but my general now has the trait 'eagle taker' which gives +1 to command. the city the eagle is in however, doesnt appear to get much. i could be wrong on that however. "

Later discussion indicated that the city the eagle in becomes a major target for Roman attacks.