Spiders (EDH / Commander)

This is not a good deck; it's mono-green spider tribal with a Fog subtheme. Many of the cards in this deck are included for flavor. There are also better mono-green tribes you can build; Spiders as a tribe do better in Golgari or Jund colors (not that Spiders are a particularly good tribe even in those colors), and better generic tribal support exists that isn't in this deck for various reson.
It was fun to build, and it's a bit fun to play, and that's all I intended with the deck, so by those standards, it's a complete success.
The primary issues with the deck are inconsistent card draw, lack of tribal synergies, very little removal or interaction, and no clear win-cons.

If you want to improve it, you can:

A. Choose a new commander. Arasta of the Endless Web is not a good commander; as a card she is completely reactive (by which I mean she only does something if your opponents do something first) and being mono green really hurts what little tribal support is available for spiders. Ishkanah, Grafwidow and Shelob, Child of Ungoliant both seem to be very good commanders and give access to black, which gives you access to much better card draw, removal, and some more efficient creatures. Ishkanah is a good graveyard-lifedrain focused deck, while Shelob is an actual spider-tribal commander. Thantis, the Warweaver would give access to red as well as black, though adding red doesn't really help the spider theme much and Thantis doesn't do anything to help spiders either. Still an option though. If you do change commanders though, I would recomend Arasta to be in the 99 somewhere, she is a good card that is capable of producing tokens.
You could also make it a Selesnya toughness matters deck. There are two selesnya spiders you can use in Ancient Spider and Pale Recluse if you really want to keep the spider theme (not that they're particularly good creatures though). Trostani, Selesnya's Voice would work well as a new commander. Making it Selesnya would also give access to more card draw and removal.

B. Remove the tribal focus. As I said, Spiders aren't a particularly good tribe (even with the LotR cards, there's not much to go with), having no specific tribal support except for one Arena exclusive card (Ishkanah, Broodmother) and Shelob, Child of Ungoliant (unless you want to also count Arachnogenesis as tribal support). Without being a spider tribal deck I don't know why you would continue to use Arasta as your commander; as I said previously she doesn't make for a good commander.

C. Be willing to spend more money. As it is currently, this deck already has some expensive cards in it, but if you want to keep it mono-green spider tribal, cards like Vanquisher's Banner, Herald's Horn, Kindred Summons, and Cavern of Souls would help the deck. Sylvan Library, Craterhoof Behemoth, and Doubling Season would also help a lot, but they all ere towards the more pricey side of things, and since I don't already own them, I didn't include them. Do note that these aren't all the expensive cards worth looking at and these cards alone won't fix all the problems of the deck, but it would allow you to keep the commander and the theme while improving the deck.

D. Remove flavor/change or remove the subtheme. A lot of cards are included because they featured a spider in some way as I previously mentioned. They don't help with the synergy of the deck, but they look nice. The only reason there's a Fog subtheme to this deck is because of Terrifying Presence. That's not to say that the Fog subtheme is completely bad, as it does help to compensate the lack of removal and boardwipes in mono-green, but there's probably better cards or subthemes to make use of (the actual viability of Fog effects is a discussion for a different day).