Kids These Days by Babblecat  

Loose morals

One day an indeterminate time since Ungoliant the Great arrived in some scary mountains, spiderlings begin appearing in the caves which the gargantuan demon-spider did not birth, making her a grandmother, sending a chill down her non-existent spine. Spiderlings do not spontaneously develop out of the ground, even she cannot produce them out of thin air, which means monsters are breeding which are not her. Of all of her children, she suspects Shelob most of all of being the most degenerate. It's definitely not Bob who's breeding. It's enough to tempt her to eat all of them and start over.