Haymitch is your person to talk to.
The main Mobs there are spiders. Of these, there are four varieties: the Splitter Spider, which spawns two silverfish when it dies; the Weaver Spider, which attacks the player with webs; the Dasher Spider, which has increased speed; and the Voracious Spider which has 1000 Health.
There is also a mini "boss" called the Brood Mother which spawns at the very top of the den. It drops a spider talisman when killed. Most spiders spawn near the center the island. Skeletons spawn mostly on the ground level, although occasionally they will spawn with a spider to form a Spider Jockey.
Slimes spawn on grassy areas while it is raining. It is recommended to take good armor because these mobs are pretty strong.
Climbing the Spider's Den leads to a Launch Pad for the Blazing Fortress.
There is also a Launch Pad to The End.
Like everywhere else it has a few Fairy Souls dotted around.