Passive

When [Nidus] dies, he instead become Undying, giving him invulnerability for 5 seconds and consuming 15 Mutation Stacks.

Must have 15 Mutation Stacks in order to activate.

Abilities

1. In this build Virulence is used mostly to build stacks, however it can deal substantial damage on regular starchart or on SP if the enemies are armor stripped.
2. Larva is a great CC tool that provides excellent grouping, especially with weapons that have punch through, like the [Nataruk] and [Felarx].
3. Parasitic Link is an absolutely crucial part of his kit that makes him effectively immortal when active, especially when it stacks with [Adaptation], giving him around 99% DR.
4. I chose to swap his regular 4, Ravenous with Tharros Strike as I find Ravenous to be not that useful in most missions, whereas Tharros Strike provides cheap, strong armor stripping and a little bit of healing too.

Mod Choices

In this build we are aiming to meet a couple of thresholds. We want to have at least 200% ability strength, as this maxes out his DR from Parasitic Link at 90% and gives him 100% Armor Stripping with one cast of Tharros Strike.

The high range is technically optional, but provides significant QOL improvements in his gameplay, such as longer Virulence range for easier stack building, longer Larva pull range, longer range for his Parasitic link, and longer range for Tharros Strike to hit multiple targets from a farther distance.

I would not suggest running him at base or negative range, as being able to move around while using Parasitic Link is very important.

Having negative efficiency is completely fine, since we are running [Hunter Adrenaline] you will be receiving energy all the time as long as you are taking damage.

Archon Shards

1 Crimson Archon Shard set for +10% Ability Strength
1 Tauforged Crimson Archon Shard set for +15% Ability Strength
2 Azure Archon Shards set for +150 Armor
1 Tauforged Azure Archon Shard set for +225 Armor

This build does not really benefit from ability strength over 200% that much, so you can run it with just 2 regular Crimson Archon Shards rather than 1 regular and 1 Tauforged. However, I would recommend as much armor as possible, and therefore running all 3 Azure Archon Shards as Tauforged would be beneficial as it does increase his EHP and add to his overall tankiness.

Typical Ability Cycle

Cast Larva into a crowd of enemies and use virulence to build up at least 1 stack, then cast Parasitic Link for DR.

Continue to cast 2 to group up and cc enemies and either build up stacks with Virulence or simply kill them with weapons, using Tharros Strike on any armored enemies.

Recast Parasitic Link whenever the timer runs out.

Additional Notes
I find myself rarely relying on his Undying Passive, since the DR from both [Adaptation] and Parasitic Link makes it very hard to die in the first place. However this passive is still incredibly useful, and not something to forget about. Building stacks with Virulence is always good for [Nidus], so there's no reason not to utilize this tool.