I love the way Ariel looks on Ais, it reminds me of the MC in Kaze no Stigma. Ironically in that series, his (Kazuma Yagami) wind can also become black once he gives into revenge (just like Ais with Tempest Avenger).
All of Vena Dis' magic is pretty awesome and OP especially the one where she sets fire to whatever she deems a heresy, Bache's magic is pretty cool too.