Combat Axe Havoc Arbitrator
 Arbites Build - Warhammer 40k: Darktide 

As great as the new arbitrator weapons are, the Rashad Mk III Combat Axe is extremely synergistic with arbitrator's kit. Historically, combat axes haven't fared well in high havocs because the increases to enemies' health precluded important breakpoints for horde clear with brutal momentum. With The Emperor's Fist and Target Priority, we reach 1 shot headshot breakpoints for combat axe light attacks against bruisers (Dreg Bruiser, Scab Bruiser) all the way to Havoc 40 (technically, we need a few % additional damage from buffs at Havoc 35+, but you will reach these BPs with any one of 10 or so high uptime buffs you'll have running). 

Attack patterns

The combat axe is very easy to use. For hordes, spam light attacks while aiming slightly above head level to score weakspot hits. Against elites and monsters, chain heavy attacks and try to hit the weakspot. Combat axe heavies have an insane amount of stagger on weakspot hit. For context, against carapace, shield slams with Branx Mk VI Shock Maul and Suppression Shield have a base weakspot stagger of 34, while the combat axe's heavies have a base weakspot stagger of 44. You should be able to reliably stagger crushers out of overheads with a heavy attack. 

Ranged choice

The ranged weapon is really your call. But, I'd strongly suggest something focused toward specialists (as we already handle hordes, elites, and bosses very effectively in melee). Exaction Mk VIII Exterminator Shotgun, Zarona Mk IIa Quickdraw Stub Revolver, Godwyn-Branx Mk IV Bolt Pistol, and Judgement Mk IV Subductor Shotpistol and Riot Shield all work fine in my experience. 

Blitz choice

Voltaic Shock Mine with grenade regen from Lone Wolf is an extremely powerful defensive combo. With three charges, when things go sideways: you can shut down all nearby enemies for a full 45 seconds (and it will only require 2-3 minutes before you can do this again). Huge packs of melee elites are completely trivialized by this setup, and you can make it significantly easier for the team to focus monsters/bosses during hordes.

Ability choice

Nuncio-Aquila gives us a strong buff/debuff combo. Given the aforementioned strength of Voltaic Shock Mine against melee elites, I would suggest mostly saving this for bosses and when under significant ranged fire. 

Both other abilities are totally viable if you feel comfortable defensively already.

Keystone choice

Execution Order is the clear pick for us. We waste the dog damage buff, but +15% damage and +10% attack speed are great. Additionally, we get a huge bonus for crits (on a weapon with a solid finesse multiplier on heavy attacks), faster ability recharge, and exactly as much rending as we need to hit 100% damage vs carapace.

Terminus Warrant really wants you to be using your ranged weapon against hordes to generate stacks quickly and ammo tends to be too tight for this in havocs.

Forceful requires spending at least a point on a talent that's not very useful for us and gives bonuses to impact and cleave, which are also not very valuable here (the former because we can already stagger everything staggerable with heavies, and the latter because a) we cleave via brutal momentum, and b) %cleave is bad with combat axe anyway because its base cleave is very low).