The highlands, which are found by the godlands, are full of enemies that can drop the Spider Den. Droppers around this area include Urgles, Undead Dwarf Warriors, and Werelions. Cyclops gods also drop this portal, but they must first be sought out in their castles.
By far the most popular reason to run Spider Dens is to obtain Healing Ichors, which are 50% more powerful than HP pots and are useful for players at all levels to keep around. Arachna and the spiders (not hatchlings) drop them.
Spider Dens also provide decent amounts of exp and two mediocre UT items for leveling players and are rather easy to rush straight to the boss. The Poison Fang Dagger is preferable to several low-tier daggers in terms of damage, while the Spider Eye Ring has a reactive proc that, when the player is hit has a small chance to slow enemies within 3 tiles, which may be a useful tool for rushing. The only other reason to run this would be to practice the bossfight patterns of the Crawling Depths, an actually worthwhile boosted version with good drops.
The dungeon’s layout is fairly linear, with a series of rooms that must be passed through in order to reach the room where Arachna the Spider Queen makes her lair there is only one passage way so it isn’t hard to find your way if rushing. Pay attention to your HP bar: these rooms are filled with some annoying spider minions that you may wish to eliminate depending on how strong your def and health are. Even more annoying are the green eggs, which hatch into more spiders if shot or approached. It is usually best to avoid these, and using a piercing item, such as a bow, is useful.
Upon entering the fight with Arachna, she will spawn a spiderweb around her that does no damage but inflicts the Slowed status effect. After this, she becomes vulnerable to damage and you are free to attack her.
If you have a damaging ability, spam it for a quick battle; Arachna has low HP and def, so the fight is not usually very challenging. She does have an annoying habit of running away while shooting (much like players do); her bullets are especially annoying given the web’s slowing influence, which makes them more difficult to dodge, as well as making it more difficult for Melee and Katana users to pursue her. If you have a hard time with her while she flees, wait for her to go back to the center of the room before you strike.