User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-16281343-20150719045603

Issei was made to forcefully drink Dragon's blood. This made his body react and dragonify. Skin hardened, eyes changed, fangs grew from his mouth. His face basically became that of a Dragon's.

Rizevim did this in front of Issei's parents and revealed Issei's secrets.[/SPOILER]