Can Daughter of the Void's legend ability be used on an opponent's turn, or does the reaction speed only allow it to be used at particular points during your own turn?

Ruling: Reactions that add energy/power can be tapped any time when you need to pay a cost for a spell/unit you are casting, including on your own turn.

Nuances:

• Reactions that have "add" resolve immediately and do not start or interfere with a chain