The shadow spider is a level 46 monster. If a player attacks this monster or the monster attacks them, it will drain their Prayer points by half of the current amount, rounded up. They can never take away a player's last prayer point. When players reach a combat level of 133, they cease being aggressive, unless they are found in the Wilderness. Several of these monsters are found near the entrance of the Waterfall Dungeon. Like Jungle spiders, they do not show on the minimap.
Before random events were discontinued, a glitch could occur involving the Spider's prayer draining ability. If you got a random event, and managed to complete it, the spider could continuously drain your Prayer even without it attacking, until it reached 1. The speed of the drain was 3 points per second.
A year after Runescape was released, there was a bug where shadow spiders prevented potions from running out.