What is the statue Hyakkimaru prays to? Hyakkimaru's mother prays to a statue without a head. What is this statue without a head?
The statue that Hyakkimaru's mother prays to is Goddess of Mercy. The Goddess of Mercy statue had a head before Hyakkimaru was born.

"The Goddess of Mercy statue sacrificed its own head at Hyakkimaru's birth so the child would not lose his head in his father's bargain with the demons.", 
(Reference - Notes, en.wikipedia.org/wiki/Dororo_(2019_TV_series))

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