Obtaining this item is quite a challenge, since it's hidden in a secret room with a relatively difficult jumping passage.
Like the item description suggests, it's use lies in charge attacks. Whenever a charge attack is fired, the projectile will leave a violet poison cloud, that continues to damage enemies that stand within it. While the poison itself seems quite unspectacular, the ticks are quite effective.
When using a fully charged attack with three projectiles, all three will leave a trail. Furthermore, damage from multiple poison clouds will stack and therefore deal triple the damage when the enemy stands in all three.While the item is rather nice, the effective range of the paper seals isn't that good. However, you can combine Edea's Statuette with Marine's Symbol, filling the entire screen with poison. Since the poison does no damage to you, this can prove to be a nice way to deal with enemies and slow bosses alike.