The Necromancer is a level 456 Boss with 10,000 Hitpoints. The best effective methods of dealing damage include Ranged or Magic.
The Necromancer will drain your Prayer, so it is highly recommended that you bring Super Restore Potions and if you can wield the Captain American Shield with your Weapon of choice, this will work in your favour as the Captain America Shield will help to restore your Prayer.
You're not allowed to bring or use Owner Cape, Icy Glaive or any other Custom Donated item to kill The Necromancer.
How to get there
To get to The Necromancer use the Boss Teleports, scroll to the bottom of the list, click on The Necromancer, then click on the Teleport button just below the picture of The Necromancer.
Since the The Necromancer is a boss, it is included in the Soul System, converting all common and uncommon drops into souls. The Barrelchest, along with all other NPC's in Dreamscape, drops the Infinity Key and Immortal Stone Fragments