Were you looking for the mystery web, a rare drop of Mirkwood Spiders?
|“||He had picked his way stealthily for some distance, when he noticed a place of dense black shadow ahead of him, black even for that forest, like a patch of midnight that had never been cleared away. As he drew nearer, he saw that it was made by spider-webs one behind and over and tangled with another.||”|
–The Hobbit, Flies and Spiders
Like normal cobweb blocks, these blocks slow down the player and other mobs; but unlike cobwebs, neither shears nor most swords can break the Web of Ungoliant any faster than you can break it by hand. Not even silk touch shears can harvest it. If you get caught in one, you will have to break it slowly or force your way out slowly – and while stuck, you remain vulnerable to the vicious giant Spiders of Mirkwood and Mordor.
A little-known fact is that the sword Sting is effective at destroying these webs – however, since this sword is currently unobtainable, this fact has little relevance
The most effective way of getting out is by placing a bucket of water next to the web which destroys the web instantly.