Given the requirements of single target necrotic dual wielding damage to 30, I'm going to explore the theme of Death:
Death is an immensely powerful and acrobatic warrior. He uses his tremendous agility to climb, wall run and leap large distances. He primarily wields the shapeshifting scythe Harvester or other shapeshifting scythes when needed. He is capable of wielding a wide variety of secondary weapons, from bucklers, armblades, gauntlets, and claws to massive maces, axes, and spears. The Horseman also wore many different pieces of armor though he eventually obtained the full suit of Abyssal Armor. He also makes use of his Phantom Horse, Despair, as a steed and his crow companion, Dust, to search for the correct path. Death also makes great use of his superior intellect to solve puzzles that unlocks his path to redeem his brother's name.
While his description has him wielding a scythe, harvester, most illustrations of him have him dual wielding because of coolness.
When we consider these requirements, of a highly-mobile enemy be visibly dual-wielding against a single target with a focus on necrotic/death themes, the answer is "half-elf avenger"
Specifically, a Revenant (half-elf) Elemental Initiate Avenger Soul Reaper (catchy, no?) Keeper of the Everflow. You're a dead guy channeling the avatar of death, becoming a small death, and then a larger death, giving death swiftly to people.
Calling the "weapons" death uses Sickles is an insult to farm implements everywhere.
Is not the cutting edge favoured by Mr. Death, save for narrative purposes. We'll call them waraxes, as the design seems to have them as a cross between an axe and a pick. Obviously, in-game we'll refluff them to "Death Sickles"
This build is designed around a Paragon start.
At level 1, you'll be using a scythe or other military two-handed weapon reflavoured to be a "death scythe."
Ki focus expertise means your "everything into necrotic damage" Death Strike ki focus fits your theme. And, in case of undead, you can turn it off, and you'll be fine.
By 11, you'll have the half-elf soul, versatile master, two-blade warrior, adept Dilliante on top of ki focus expertise, which means at 11, you can cheerfully wield two waraxes glowing with necrotic energy into battle, necrotic energy that no one can ignore.
Everything else can basically be taken out of the avenger handbook with an eye towards refluffing. When looking for build advice, look towards suggestions for the "critvenger."