Isma's Tear

Function
Acid shall be repelled.

Swim in acidic waters without coming to any harm.

Allows the Knight to swim in acid without taking damage.

Usefulness
Albeit very useful the Isma's Tear is not necessary to complete the game, it allows access to the following things: It is also useful when exploring Fog Canyon and fighting Uumuu.
 * A Hallownest Seal found on a corpse in Greenpath.
 * The Shape of Unn charm.
 * It lets the Knight access Fog Canyon through Forgotten Crossroads and the acid pool near Leg Eater. This is one of the two ways to fully explore the area. The other is by using Shade Cloak
 * It allows access to Queen's Gardens. However, an unintended way to do it is by using Crystal Heart.

How to Acquire
Located in Isma's Grove in the Royal Waterways. Requires Crystal Heart and beating Dung Defender in order to access this area.

Trivia

 * In the game files, this ability is referred to as "Acid Armour".
 * Dung Defender refers to it as "Isma's Blessing".