The Spider's Den is an early to midgame island for players to kill spiders. It has the first introduction to bosses in skyblock.
The Brood Mother is a relatively easy boss to kill, as most of the playerbase can kill it in 1 hit. It drops several items that help with game progression.
Arachne is a midgame boss in which players must obtain Arachne Fragments from killing spiders in the den and placing them at the altar in the middle to spawn the boss.
The Spider's Den branches out into the Blazing Fortress, and the End.
Image taken by me, created by the Hypixel Team