[@Bishop]

While it's true that Ungoliant and Morgoth had a... falling out, shall we say, and that the spiders are an independent evil to Morgoth... you're still taking a lot of liberty with the lore. The spiders are evil, caring nothing more than to kill, eat, spawn, repeat. One wouldn't approach an Istari for aid, and no Istari would even think of helping such a beast. Unless said Istari had turned from his true nature as Saruman did. As for the power of transformation, I have doubts whether a wizard could give another creature such a power.

Regardless of all this, how on earth would it fit into the roleplay? It can't speak to anyone, it shouldn't have any want to do anything good or kind in the world, and as soon as it's true nature is revealed, it would be killed instantly. It's an interesting idea, but maybe for a monster roleplay - not a Lord of the Rings one.