A Charmed sack is an item used while training at The Nexus. It can be claimed by talking to Ysondria, or by right-clicking her and selecting "Get charmed sack". It is used to store some of the corruption of the Grotesque and then emptying its content into a bowl on one of the pillars.
Initially, it can hold up to 15 corruption at a time, but after purifying enough corruption, it gains the ability to hold up to 18, 22, and eventually 25 corruption at a time. It retains this property after being destroyed and reclaimed (although all contents are lost).