A Soul is an entity you can obtain by slaying any mobs with any scythe power attack or SoulSteal -enchanted weapons. You can collect them with empty or soul-filled Essence Keeper or Essence Vessel.
When you reaped a soul, right-click on it with empty or soul-filled Essence Keeper or a Essence Vessel to collect it.
Farming souls is easier than farming bloods, since a soul does not need a special block nor does it get blocked by any block.
The easiest way is using a monster-grinder (especially a Soul Cage grinder), just wait until there is a lot of mobs and use the Reap ability, all of them will die (except infernals), and it can fill your Soul Crucibles very fast.
A harder way is by finding a battletower and just battle using a SoulSteal enchanted weapon, you can get over 5 souls when you 'finish' the battletower.
In multiplayer, having one person to kill mobs and another to bring a lot of Essence Keepers and collect the souls is also a preferred way.
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