User blog comment:B214/General Discussion/@comment-24098614-20170401200954/@comment-82.60.161.59-20170401202301

they are absolutely not invincible and they can be killed. Holy power and dragon slayer power are indeed dangerous for them, they can only increase their endurance to withstand them, but they will always be dangerous for them. Obviously it also depends on the quantity of power of the one who use them. For example if Issei is hit by Arthur with Caliburn he will for sure be badly injured considering the power of the sword and of the wielder. They have an incredible power but it doesn't last long, if they are not able to finish their opponent in that amount of time, they risk to end their energy and be killed.