I've come up with a spider lord but not a legendary spider per se:
Arachnia, Mistress of the Web Legendary Creature - Human Shaman Reach Spider creatures you control get +1/+2 for each other Spider creature you control and have deathtouch. , : Put a 1/2 black and green Spider creature token with reach onto the battlefield. 2/4
By the way, does anyone else here think Spiders ought to become a B/G tribe? I mean, tell me something that looks like the Arachnus Spinner couldn't be at least partially black. Not to mention the Spider's cousin, the Scorpion, is an established black creature type. You even have a Giant Scorpion to compliment the Giant Spider . That's kinda cool.
Uraberg, the Broodmother Legendary Creature - Spider Deathtouch, Lifelink, Reach At the beginning of each upkeep, put a 1/2 green spider token creature with deathtouch, lifelink, and reach onto the battlefield under your control. Spider creatures you control have +1/+3. 5/7
Mown, you may be right about scorpions in real life. I had always heard differently from people who have been around them.
However, I also determined to do digging on scorpions in Magic. It happens that there are only six scorpion cards in existence. Honestly, I dont think a tribe can be defined with only six cards, so I refute any correlations between the two tribes as being bunk based on there not being nearly enough evidence to suggest what scorpions are or aren't in Magic.
In any case, this thread has diverged far from being a spider specifically for Commander to being what can we create that might resemble a legendary. Thanks to those of you who actually put some thought into your cards. I've figured out what I wanted, and I probably won't be posting any more here.