Goliath was a Giant that was friends with Xena, but later fought against her and the Israelites, which leads to his death.
He knew Xena many years ago, when to save her his family got killed at the hands of Gareth. For this, she vowed to never fight to death against him, since Goliath lost what meant the most to him to save her.
After Goliath died, Xena promised herself that she would avenge the death of his family. She did this when she later killed Gareth.