The mystery web is a semi-rare drop of the Mirkwood Spider that is said to be the remains of its last meal.
Obtaining
The Mystery Web can only be obtained by slaying Mirkwood Spiders. It does not drop from a Mordor Spider.
Use
Once thrown by right-clicking, it will drop various items (some useful, some not so much) such as leather armor, bones, swords, gold nuggets, and daggers, or even a slice of melon (this is one of two ways to obtain melons in Middle Earth, the other being a rare spawn in Far Harad Jungle).
However, the player should never think that a mystery web marks the end of their fight, because the Mystery Web can also spawn Mirkwood Spiders.