The summoning he was aiming for didn't use his pure strength or energy brimming inside his body to call upon his master, but it uses his soul force; a very rare and unique form of strength that usually needed a long time to be replenished.
Any weakening to the soul force for anyone would result in him getting greatly weakened. That was the reason behind his hesitation, but as the system was so confident in helping and protecting him, then he had to give it a shot.
"I will bleed myself, but not for your sake, but for my own summoning. When I give you the signal, then you can have the blood you want," Arthur said as he slowly moved his claw to injure his leg, creating a long gush of fresh red blood oozing out from the wound.
He knew his body would heal spontaneously if not for that sealing power and oppressive force he was subjected to. And he didn't care, as he needed this blood now to do the needed rituals.
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