So vividly did he imagine the massive head turning his way and opening its huge maw that he had to blink to ensure it didn't happen.
He imagined the dragon's massive head very clear and vividly. 
mnli hypothesis: He imagined the dragon's massive head very clear and vividly.  premise: So vividly did he imagine the massive head turning his way and opening its huge maw that he had to blink to ensure it didn't happen.