I am sick and tired of people suggesting wrong builds for my second most beloved frame after [Baruuk] (same applies here, wrong builds everywhere + stat stick ... Baruuk is NOT using external melee booster or Riven mods! Only gladiator mods are valid!) - Harrow.

People like robot-chicken Grind Squad and Co. are suggesting more shields, using melee as your primary damage dealing weapon (rakta dagger in this case), using Arcane Aegis and Barrier and what not.
All wrong. You won't survive any crap passing Lv 150+ since you your actual damage mitigation is worth nothing when you focus on shields!
Shield gating is not helping in this case! You are merely attracting every enemy to shoot at you, even if you don't find any hints about it at the wiki! Like the example of Baruuks Desolate Hands blocking every status procced effect (health drain, toxin DoT, fire DoT, etc.) and it also isn't noted by the wiki nor DE.
I've also featured many successful and popular builds on Warframe-Builder, but the Host got onto my nerves with his non-stop steam-ads (with random html-IDs) posting about a crappy text-based game he made.

So, you can trust me, if I say, that this is with a high margin the best build you can get for your [Harrow]!

Short demonstration:
[Warframe] Harrow - Destructive Kuva Nukor Chainer - Short Demonstration

Lets start, shall we?

Why no range at all? Because it is worthless. You will proc and replenish your shields with your +100% status chance radiation based [Rakta Dark Dagger], not with his first ability, Condemn!
You won't need [Rolling Guard] (3s invulnerability), because you have Vazarin! You heard it right, we are going to use Vazarin, as this is the most powerful combination with [Harrow].
Also, no one uses [Harrow] as a support in regular game modes. There is no need for a ranged build in this case. You are not going to camp or such with randoms/friends!

Zenurik is obsolete due to his third ability Thurible, and you won't need to keep it up running indefinitely. Only when you are near your 100 energy pool mark!

Naramon is also obsolete. We are not focusing on underpowered melee strikes. We are here to deal tons of headshot damage with a simple chain reacting build. You will have 100% status chance anyways, so there is no need for Weeping Wounds guaranteeing a rad-proc.

You are going to use a [Kuva Nukor] in this case. This is where Arcane Precision and Pistoleer becoming bloody effective, not only efficient! You will melt down lv 170 Gunners with ease. The sheer fast fire rate will blast off your ammo, but you can counter that with either Pistoleer (recommended with ->), Ammo Mutation mod or [Carrier] (Prime).
Bonus on [Kuva Nukor]: Innate radiation element = ultimate CCing.
Kuva Nukor
Addition since Galvanized mods introduction: Keep in mind that the [Kuva Nukor] got a nerf in terms of max chaining on enemies from 4 to 2, which really is not impacting the overall damage and kps.

About the [Rakta Dark Dagger], it is simple. Fast attacking only radiation proccing weapon. Not needed for anything else!
Rakta Dark Dagger

You can also use a Sniper (preferably Rubico Prime) with Sharpshooter, if you are having a hard time casting Thurible for more than 10 energy / kill base value.

Gamestyle:

• When you get around +100 energy at your energy pool, then you cast Thurible. You should get a 10 energy/kill base value, before you start wrecking things.
• You will gather up more and more enemies. Keep yourself save by attacking the enemy with your Dagger. Replenish those shields and keep them in overshields.
• Kill enemies with a headshot, to gain an increased amount of energy.
• Cast Covenant and gather at least 1000 accumulated damage (this is enough to reach 50% crit chance on this buff).
  -- While getting damaged, max out your overshields and cast Penance! Very important! Regain shields + overshields as fast as possible. It is advisable to use Magus Lockdown with Vazarin combined - being invincible for few seconds + pinning down enemies for few seconds is the real deal of survival on every frame!

And now? Kill your enemies with speed and damage values beyond your imagination.
Covenant has its augment "[Lasting Covenant]" included, which means, each headshot kill will give you 7ish seconds on top of your current ability duration. It should cap out around 90 seconds of buff duration. This will never run out, if you simply aim for heads.
As you are using a [Kuva Nukor], you will weaken and more often kill your enemies with the chain reaction, even if you don't want to, which makes killing enemies via headshots in larger groups kinda gets tricky.

Infested are quite glitchy and wonky when it comes to headshooting stuff.
Sometimes you will have hard times hitting a head, other times you will chain-headshot the crap out of a bunch of enemies which will result in time-capping your buff's duration instantly at once.

Hope that you guys will enjoy this build.
Happy hunting, Tenno.

Best regards,
Pacheon

Update:
Weapons have been updated accordingly to the newest Galv-Mod additions and [Berserker Fury] + (Primed) [Fury] nerf.