This is my Steel Path Xaku build. It provides amazing damage as well as amazing utility. Disclaimer: Use caution using this build against high-level Corpus - the main DPS source cannot target enemies inside of Nullifier bubbles and walking inside a Nullifier bubble ends all of the abilities the build relies on, which are very expensive to have to recast all at once.

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Skills:

Grasp of Lohk:
Our main source of DPS. It disarms nearby enemies and summons floating, auto-targeting copies of their weapons that follow [Xaku] around. The number of guns it can summon scales with Ability Range and the damage of each gun scales with Ability Strength, so we're specced heavily into both. 235% Range and 194% Strength gives us a total of 14 guns, which each fire 1.2 shots per second and do 97*EL damage, where EL is the level of the enemy the gun was stolen from. With level 100 enemies, this gives us about 163k DPS.

Notes on GoL: This skill can be recast if not at the maximum number of summoned weapons to summon additional ones, and can hit already disarmed enemies. Each gun has its own duration. The type of weapon stolen from an enemy does not affect the summoned copy's stats. If this skill hits a target without a gun, it summons a copy of whatever Primary Xaku has equipped. Does not require line of sight to steal weapons, but does for weapons to shoot enemies.

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The Lost:
Provides us with several amazing utility sub-skills:

[Gaze]:
The most important one by far. Single target skill that traps an enemy, stunning them and making them invulnerable. While stunned, they project an aura that reduces the current armor and shields of enemies inside it. With 194% Strength and 235% Range, the aura has a radius of 28.2m and a defense reduction amount of 97%.

Notes on [Gaze]: Up to 2 Gazed targets can exist at a time. If recast while 2 targets are already affected, the oldest target is set free. Requires line of sight.
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Accuse:
Summons a void fissure that turns nearby enemies into allies. With 194% Strength, we can affect up to 15 targets.

Notes on Accuse: This skill can be recast if not at the maximum number of targets to convert more enemies. Each converted enemy has its own duration. Converted enemies can still be hurt by Tenno. Does not require line of sight.
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Deny:
Big burst beam skill that also suspends enemies it hits in stasis. Damage scales with both Strength and number of guns active from GoL. With 194% Strength and the maximum number of guns summoned, it does 116k damage to anything it hits or passes close to.

Notes on Deny: A little bit tricky to hit with. Also resets Sentient resistances. Has infinite punchthrough for enemies but is blocked by terrain.

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The Vast Untime:
The heart of the build. With 194% AS It does ~2.3k damage in a large range, slows anything it hits by 48%, and applies a 50% weakness to void damage, which translates to a 1.5x multiplier to all of [Xaku]'s damage abilities. It also increases the 25% dodge chance and AoE damage reduction from their passive to 75%, gives them 20% bonus move speed, and stops the duration of all of their other skills from decreasing. The skill has a short cast animation during which your skill cooldowns will decrease, but you can easily keep the same set of guns active for 15 minutes if you don't let TVU drop.

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Spectrorage:
Our Helminthed ability, replacing [Xata]'s Whisper. It creates a ring of rotating mirrors that [Charm] nearby enemies into entering the ring and attacking them. The [Charm] radius is twice the ring's inner radius. While Spectrorage is active, enemies within range will not attack anything else. We also run this skill's augment, Sprectsiphon. It causes enemies charmed by Spectrorage (including ones who are not inside the ring of mirrors) to have a 50% chance to drop energy orbs. This skill is our one-stop shop for CC and energy management.

Notes on Spectrorage: Duration is not paused by TVU. Number of mirrors summoned is affected by Range, which changes the skill's radius because the ring is expanded to fit them.

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Arcanes

You can pretty much use whatever here. My personal preferences are:

-Arcane Grace. The obvious choice for tankiness with Umbral Vitality

-Arcane Energize is great for giving everybody energy from the Energy Orbs Spectrosiphon spawns.

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Focus School

You can pretty much use whatever here. My personal preferences is Zenurik because Energizing Dash is great if you're short on energy and need to recast TVU.

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Play Style
This build is extremely strong and fairly easy to play, but requires a fair amount of attention. TVU should never not be up. With this build it has a 38.75s duration, so it'll last a while but you need to make sure it doesn't go down, as that not only decreases your defenses by a lot but also lets the durations of your other skills start decreasing. So keep that up, summon your 14 guns, and melee or parkour your way around the map while they melt everything. When you run into clumps of enemies, you want to cast Spectrorage both for the taunt and the energy sustain.

For endless or point defense missions, you want to use [Gaze] and Spectrorage to set up a large area where enemy defenses are massively decreased, enemies are taunted, and enemies have a bonus 50% chance of dropping energy. Otherwise play is the same.

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Tips And Tricks

-I prefer to manually turn off TVU and immediately recast it when it only has a second or two of duration left. This minimizes the downtime.

-For endless missions, you periodically want to find somewhere safe, drop TVU, let your guns expire, and recast GoL. This is because gun damage is based on the level of the enemy originally hit by GoL and in endless missions enemy level increases over time.

-Always cast TVU before you start summoning guns. This will maximize their duration.

-TVU is great for breaking destructibles in a huge radius.

-I like to run Rakta Dark Dagger with this build for bonus overshields and free energy, although it is absolutely not necessary.

-May add more as I think of them

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Build Variations

-If [Xaku] has 200% Strength or more, [Gaze] will remove 100% of enemy shields and armor. Note that this will change the defense type of some enemies. This is the reason I run Growing Power as the Aura - you can choose whether or not to 100% strip. If you don't want to forma the Aura slot, you can run whatever you want. Note that Corrosive Projection stacks multiplicatively with Gaze, so it is pretty much useless on a high-Strength Xaku.

-[Preparation] is a huge QoL boost and I highly recommend at least trying it, but it is not necessary. There are plenty of other great Eximus mod choices, although of particular note are [Cunning Drift], which will grant you an extra summoned gun for either choice of Augur mod (see below), and Power Drift, which will increase gun DPS by ~5%.

-After a lot of experimentation I have found that I prefer [Umbral Vitality] over other choice for an 8th mod because the survivability boost is massive and it comes with a bonus 11% ability strength by way of Umbral Intesify. However, this build can operate as a glass cannon without it. Other options are:

-[Augur Reach]. Adds an additional gun, and experimentation shows that the extra range translates to a roughly 22% increase in Spectrorage's radius, which means a roughly 48% increase in area covered by both the ring and charm region.

-Augur Secrets: Allows Xaku to hit 100% armor strip without GP. Also increases gun damage by about 1%.

-Obviously this version of the build is very expensive, including requiring an Aura Forma and an Umbral Forma. There are a number of ways to adjust it to make it more budget-friendly. Here's a version that works just fine and only requires 2 regular formas: Xaku: Gun-o-mancer on a Budget.