Yeah, when I started out, I did what a message on the ground told me and tried to backstab the Black Knight, took out at most 15% of his health, and then ran away.
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I'm definitely impressed you killed him before even the Taurus Demon.
Make it a thing to read
every item description, man. You learn a lot doing so, about mechanics, lore, next steps, and so on. Unfortunately, if you used the Fire Keeper Soul instead of handing it to a Fire Keeper, you've lost a potential Estus Flask upgrade. Not the end of the world, though. You got the soul from the New Londo Ruins, right? I didn't get that one for a
long time. You'll be fine.