Poison Nova is a powerful attack if invested enough in. The range of Poison Nova is much greater than other novas in the game, such as the Sorceress' Frost Nova or Nova (though not nearly as large as Diablo's Fire Nova).
This spell befouls the atmosphere surrounding the caster. With an arcane chant, the Necromancer corrupts the very air we breathe and causes it to erupt in all directions. It is from years of investigation and preparation that the Necromancer can choose a poisonous gas that is toxic to the minions of evil, yet harmless to himself and his companions.
↑Diablo II Manual, page 75. Blizzard Entertainment, 2000.