Perhaps Darius has a personal vendetta related to the chariots of war, and Xena decides to help him. As they work together towards this goal, they start to care for each other. Xena might teach Darius some new combat moves and in return, Darius shows Xena the finer points of handling the chariots. Their relationship evolves from acquaintances to friends and maybe even more as the story progresses.
In the fan fiction, they could be forced to rely on each other in a dangerous situation involving the chariots. For instance, if they are trapped in a valley and the only way out is by using the chariots in a very specific and dangerous way. This shared experience would deepen their bond. Also, they could have disagreements about how to use the chariots at first, but through communication, they grow closer.
Their relationship could start with mutual respect for each other's abilities. Darius might be in awe of Xena's fighting skills and Xena could be intrigued by Darius's knowledge of the chariots of war.