Having prior knowledge from playing Demon's Souls extensively, I was very careful when I got into the possession of a 'Fire Keeper Soul'. It says the item increases Estus flask potency in its discription. It does, but only when turned in to the right NPC. Consuming it mearly grants +5 humanity and no permanent buffs. Tankfully, I knew something like this would be the case and so I googled it before doing anything with it.
Are there any other souls (or other items) that I should be weary of using just-like-that? In Demon's Souls, some souls upgraded specific weapons or unlocked powerful spells for example. Back then, I found out the hard way when I was 3/4th trough my first playthrough not being able to learn a useful miracle. I hope to avoid such 'surprises' this time around.
So if anything comes to mind, please share.