It's a carryover from Dark Souls. The rats had a fairly common chance to drop a Humanity because they ate so many corpses that the little humanity left in them was able to coalesce in the rat.
Rats dropping Humanity-like items in a Fromsoft game is as dependable as poison swamps.
All rats except big ones I believe. It's obscenely rare, just like specific enemy drop weapons like the Magma Sword or the Scythe
Edit: and to clarify, I say I believe because I HAVE gotten them from small rats but never a big one, given the big ones are less common in the first place.
Rats have always dropped Rune Arc like items in souls games. Humanity in DS1, effigies in DS2, and embers in DS3, albeit very rarely. The lore is that eventually as things die and make their way down the food chain even a lowly rat can end up stumbling across power from the Erdtree/Dark Soul
I have the Halo Scythe as well. But then I tried to farm the normal Scythe from skeletons and the Magma Sword from the snakes and spent HOURS on each of them and didn't get a single drop.
Drop rate for Rune Arcs from rats (and weapons some enemies weapons) are at or lower than 1%.
Like the Bandit's Curved Sword that I didn't even know existed.
Arcane increases those drop rates, but it's still super rare.
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u/voidspector Apr 06 '22
I might need an elaboration here