Shadow nihil icyenic creature created by the god Zaros through genetic experimentation with Demons and are first encountered during Fate of the Gods.
Nihil are one of the most powerful non-boss monsters in the game and you must be well prepared before fighting them. They have very high defence and extremely accurate attacks and therefore it is recommended to use stat-boosting potions and/or prayers to break through their defence and increase your own. It is also highly recommended, if not required to wear the Shard of Zaros when fighting the Nihil as they are aggressive without it and you will be killed very quickly after being attacked by all of the Nihil in The Pit. Furthermore, each Nihil is capable of using a special attack based on the Ancient Magicks spells that share their name.
The Shadow nihil signify their special by hissing before a shadow will appear under you and hitting 500 3 times, however this can be avoided by moving when it hisses.
You can persuade Kuradal to assign Nihil as a task at level 76 Slayer for 50 reward points.