Bathory was, along with Hellhammer/Celtic Frost, the first Black Metal band. A lot of the influence on Black Metal people attribute to Venom is actually the work of Bathory. Quorthon said he wasn't influenced by Bathory, but by British Punk and Heavy Metal bands, creating something not unlike a European Slayer.
I think that in some ways Bathory was more important than Celtic Frost, Celtic Frost was certainly influential but their technique was still quite rooted in traditional Heavy Metal while Bathory was more unique at the time.
The Slayer/Bathory comparison is interesting because while they were both essentially Heavy Metal/Punk hybrids, Slayer was influenced more by US Punk and NWOBHM like Iron Maiden. Bathory on the other hand was influenced by UK Punk and older British Heavy Metal like Motorhead and Black Sabbath.