This is one of those weapons were a good riven makes an absolutely huge difference to the overall DPS. A good crit damage + faction damage riven will literally triple the sustained DPS, so it's well worth looking for one before the Riven prices skyrocket.

This build was made based on the datamined information about the Despair's incarnon form / evolutions, shown at the bottom of this guide:

Exilus mod is completely subjective. However, if you want to run pretty much anything except minimum rank [Pistol Ammo Mutation], you'll need to pop an additional forma on the '[Lethal Torrent]' slot.

With the ridiculous boost to crit damage on the Incarnon form, you should be aiming for headshot kills as much as possible, hence [Galvanized Crosshairs] and Secondary deadhead to make your life a bit easier. If Tier 1B evolution causes the additional blades to home in on heads, like the [Mk1-Kunai] incarnon does, then this playstyle becomes both easier and a lot more powerful.

IF TIER 1B DOES NOT GIVE HEADSHOT HOMING: Probably better to swap [Galvanized Crosshairs] for Creeping Bullseye or Primed Pistol Gambit, but I still think Tier 1B will be stronger than Tier 1A.

Datamined Incarnon Evolution tiers: Recommended = 1B, 2B, 3A (if running Dread + Hate in other weapon slots) or 3B (if not)

Tier1A - On hit: Multi-shot and ammo efficiency increase, to a max of X. Resets on weapon swap. Multi-shot and ammo efficiency transfer to next Y shots with Primary weapon on swap if movespeed is above Z.
Tier1B - [Kunai] have homing abilities and split into two new homing kunai on impact. Deal 2x damage to enemies below 50% with active channeled ability.

Tier2A - Low FOV.
Tier2B - Projectile Speed.
Tier2C - Reload Speed.

Tier3A - With Dread and Hate equipped: Multi-shot and aoe on impact.
Tier3B - Balanced Crit and Status Chance.
Tier3C - Reload from empty: Increase fire rate by X for Y secs.