This is considered a mystery. In the link below is the exact location of the first one I found in Meath :
https://prnt.sc/1302lynOn the map it's shown as a bonfire icon. All you have to do is approach the bonfire and press Y to interact. It's like the mushrooms mysteries, some enemies spawn and you have to kill them all to complete the mystery and the achievement will pop. TIP: this one was a bit annoying because the enemies were using poison and the area was a bit foggy. By using the Dive of the Valkyries SKILL I cleared them all with a shot each.
(NOTE: I'm power 425 and playing in drengr)
Edit: they all use poison and it gets even harder after each trial. The one I linked is the easiest.