IIRC Regis did drink blood before fighting Vilgefortz.First of all lets get all the facts straight. Regis was not at full strength when he fought Vilgerfortz. He was a pacifist Vampire who did not like to kill or hurt people. He refused to drink blood and that drained him of a good portion of his power. Detlaff is a natural born killer who drinks blood for fun lol. His at full power and in constant rage mode. He can turn invisible, teleport, quicker then lightning and can turn into a giant bat-demon! Lmao Not to mention His been killing for centuries. Without a doubt Vilgerfortz gets STOMPED!
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A lowly mortal human who happened to be the most powerful mage in the books, including some mages who were 500 years old.Fact:
If Vilgefortz was a boss encounter in any of the witcher games, Geralt would be able to defeat him without any help, too.
Dettlaff would totally mop the floor with that lowly mortal human Vilgefortz, particularly given the chance for rematches after regeneration. Vilgefortz will grow older and older and at some point have dementia and gout and incontinetia and and and ... while Dettlaff will just keep regenerating.