The Shadow spider is a level 66 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 105, they cease being aggressive, unless they are found in the Wilderness. Several of these monsters hang around near the entrance of the Waterfall Dungeon. Like Jungle spiders, they do not show on the mini-map.
Sometimes a glitch occurs involving the Spider's prayer draining ability. If you get a random event, and manage to complete it, the spider can continuously drain your Prayer even without it attacking, until it is 1. The speed of it is 3 drains per second.
A year after Runescape's release, there was a bug where shadow spiders prevented potions from running out.
There is currently NO way to prevent Shadow Spiders from draining your prayer points.