Sanil's Murdrum is a spell available in The Elder Scrolls III: Morrowind that temporarily increases the value of the sneak skill and at the same time concealing the presence of the caster. This spell is very good for the stealth quests.
This spell is not available for purchase in Morrowind or expansion packs. Although not intended for use by the Nerevarine, it can be acquired on the PC by using the console:
Player -> AddSpell "sanil's murdrum"