Staff is a no-brainer for a Necromancer in a zerg. It tags a lot of enemies, provides a blast finisher, combo fields, and in general AoEs are a central point of any zerg fight. You will use the autoattack as little as possible, switching to other weapon sets, skills and/or death shroud as soon as all your marks are placed.

The second set is the one you'll use to finish people trying to get outside the fight, and is much more free. I would suggest in any case Warhorn because of the useful swiftness and cripple on Locust Swarm [5] (so much that my autoattack is set on that), and I like Dagger because it is the highest DPS weapon for a necromancer on autoattack and the Dark Pact [3] plus Warhorn's Wail of Doom [4] can block an opponent for quite long, allowing the other players to join in.

The skills are almost all Wells, except for Signet of the Locust, that provides a very useful speed boost to pick up a zerg or stay in line with it. The ideal thing would be to switch it for another well (Well of Darkness being my favourite, but Well of Suffering does more damge) if you have time to prepare for the clash, but the Signet works well anyway, since it acts as an AoE health stealing, fitting the theme of the build.

The elite is Plague. Avoid Withering Plague [1] when in such form, since our condition damage is abysmal, and rather use Plague of Darkness [2] when turned for defensive purposes, and Plague of Pestilence [3] when used offensively. Remember that Plague is a great way to boost your HP, and the blind allows you to substantially avoid all melee attacks, but when you will get out of it, if not out of the mass, you will be in big troubles, so be ready to dodge and heal yourself! Pro tip: commanders could seriously love a blinding or crippling Plague following them closely.