User blog comment:B214/Volume 21 info & spoilers/@comment-207.96.205.2-20160508025110/@comment-24.113.252.213-20160509131349

I think I have an idea on how Vali gets his new Diabolos Dragon power.

Remember how in vol. 12, Siegfried and Jeanne used a special doping using blood of the Original Satans to use what is called Chaos Break/Chaos Drive, in which the Sacred Gear (and Holy/Demon Swords) fuses with the user while transforming the user into a monstrous form. Like the Juggernaut Drive, it consumes the users life span with each use.

I think Vali does the same thing with his Divine Dividing, using his Lucifer Blood, and Ophis (or Lilith) uses her power to stabalize this form so that the Sacred Gear fuses with Vali without transforming him into a monstrous form and consuming his life span.