Dracula was trying to be as calm as possible, however, in reality the Surya-Astra was rattling him from the inside out.
With the two of them fighting in space, where there was no atmosphere, Dracula was already at an advantage as the darkness of space did not refract light like a planets atmosphere did, however, despite this advantage when Regus used the power of the Surya-Astra against him from close-range, Dracula felt strength leaving his body as he felt as weak as a feather.
The modern vampires did not understand how serious the debuff from sunlight really was for the primordial vampires.
It wasn't a simple case of ' Sunlight weakened them ' or a skin disease that sizzled upon coming in contact with the sun.
The primordial vampires literally felt like someone had thrown their entire bodies into massive pools of toxic chemicals when they were exposed to sunlight and this debuff was particularly strong for their progenitor, Dracula.