As part of the alliance between Humans and the Sidhe, and in an attempt to mend the hateful feelings on both sides, many Sidhe daughters were married into populist human households. Originally, this was agreed to as insurance that the Sidhe would not betray their treaty and attack again once the threat of the Brae was beaten back. But as time wore on, the intermixing of the two cultures brought connections between the races which had not been there before. Almost to a one, the children grew to be charismatic, respected, and heroic individuals, and they became a symbol of a newfound, heartfelt truce between the Sidhe and the Humans.
Sidhe-Blood use the default rules for half-elves in the Players Handbook.