Damien decided to help the Ancestor seeing how he was the kind of man who cared about his own family more rather than the kind of man who would do anything to keep their empire safe.
If the Ancestor was concerned about the safety of the empire and his own, he would have long given up Lily for marriage to any one of the other empires and even rake in some benefits for himself.
Instead, he was ready to sacrifice his own life for Lily just like how a loving grandfather would for their grandchildren.
A few moments ago, Damien mentally conversed with Luna, "You can remove the poison essence from his body, right?"