[Shield Of Shadows] gives [Nekros] a significant boost in survivability when paired with other DR mods. We subsume an additional defensive or QoL ability to roudn out the build against various content.
Null Star is chosen because it gives an additional 90% damage reduction when you're at maximum ability duration. Dispensary, Elemental Ward, or any QoL buff also work on this build. Duration can be dropped a bit on those abilities for more range if it's needed.
Soul Punch is replaced because it's the weakest ability in most [Nekros] loadouts.
Ability Strength needs to be as close to 215% as possible to get the full damage reduction from [Shield Of Shadows] even if you only get 7 enemies in a cast.
[Enemy Radar] mods help you and your team to track down enemies. It can be swapped to any other aura to best suit the mission you're playing.
[Streamline] can be replaced with [Adaptation] if you're confident in your energy economy and want a bit more damage reduction against specific attacks. It doesn't even need to be max rank to get the full DR.
Arcane [Guardian] significantly increases your survivability against general content. It can be replaced with any other arcane to suit your playstyle.