This spider does not use its web like other spiders do. With a black body and brownish-black hair, it relies on its camouflage skill. It often lurks on a victim above doors or tighter spots, unmoving and unnoticed for long hours. In the fore-legs it holds a piece of its web (it is very firm, only a little sticky, but above all: it is black). This web is wrapped around the head of the unsuspecting victim, and it is bitten in the neck. The spider then holds the victim, so it does not scream or move too much, and waits until the poison takes effect and the victim passes out.
It’s poison is a weakening and paralysing toxin, it can put one to sleep for hours. It could be possible to harvest it.
It also has a nest - a place where the victims are deposited. It takes a few days until a victim dies (meanwhile safely webbed in a cocoon). The spider takes care that it does not run away, and regularly administers another substance: weakening and conservating. When the victim is finally dead, the body can endure for tens of years without decomposing. If the spider has enough of other food, it is even excelently preserved. (If your campaign allows raising dead, this can be useful. But a simple Speak with Dead spell can do much too.) If the spider has enough fresh prey, and enough preserved bodies, it will lay eggs into some.
The Solitaire Spider is a loner. It is relatively smart, and it can ambush its prey effectively. On the other hand, it is ready to run away if the opposition is strong, and strike when it senses weakness. It avoids attacking groups. You could make it even smarter, and let it use treasure of former victims to lure new ones.
Usage: A properly used Solitaire Spider can with a bit of luck knock out a whole group of adventurers. This is rarely necessary, only if the group does not hold together. It is useful for “vanishing” of a character, often of the last-in-the-line. This way or that way, if the character (group) does not manage to free itself, someone else can do it. Paradoxically, the Solitaire can save someone from a life-threatening situation (at least temporary… ).
DM: “You hang over that bottomless hole, your fingers slip slowly, and you realise you have left your friends too far behind…”
PC: “Help! Help… well, so I try to gather my last Strength…” (Roll fails)
DM: “Your fingers cannot hold you anymore… but in the last moment some long limb with a claw catches you!”
PC: ” ??? ummm, OK, fine…”
DM: “A great black spider pulls you out, and wraps you some black cloth around the head. While you struggle for your breath, you feel something biting in your neck…”
Note: There are a few more anatomical details below in a response to Zylithan’s questions.