Disclaimer: This build guide is far from perfect and can be improved. Feedback is appreciated to enhance its accuracy and usefulness. I also used chat GPT to make it more readable as my English isn't really that great. Thanks!

Aura:

• [Brief Respite]: Utilize [Brief Respite] as the top choice for the Aura slot. This choice aims to abuse shield gating effectively, providing valuable survivability.

Exilus:

• For the Exilus slot, you have the freedom to choose any mods based on your preference. However, [Power Drift] is recommended for its added Strength (STR), enhancing your overall power.

Mods:
1. [Umbral Vitality]: Equip [Umbral Vitality] to bolster both your Health Points (HP) and Strength (STR). This synergizes well with [Umbral Intensify], providing a balanced boost.
2. [Umbral Intensify]: Boost your Strength further with [Umbral Intensify]. The combination of Umbral mods strengthens your abilities and survivability.
3. [Primed Continuity]: Add [Primed Continuity] to [Extend] the duration of your buffs and counteract the negative effect of [Transient Fortitude].
4. [Primed Flow]: Increase your energy pool with [Primed Flow], allowing you to use abilities more freely.
5. [Transient Fortitude]: Include [Transient Fortitude] for increased Strength (STR). You can opt for [Blind Rage] if the negative efficiency is acceptable to you.
6. [Furious Javelin]: Incorporate [Furious Javelin] for a substantial damage boost to your melee attacks. The damage scales based on the number of enemies hit.
7. [Surging Dash]: Flex slot which you can change according to your preference. It helps build combos fast without relying on your stat stick melee. You can use [Stretch] if you want more range and faster slash dash.
8. Rolling Guard: This is a mandatory mod that acts as a safety net between Slash Dash casts. Combine Slash Dash, Roll, Cast Blind/Javelin/Buffs, and repeat until [Rolling Guard] is off cooldown.

Arcanes:
While you have the freedom to choose any Arcanes that align with your playstyle, the following Arcanes are recommended for this build:
1. Molt Augment: This Arcane enhances your Strength (STR) and provides an increase to the [Furious Javelin] damage buff.
2. Arcane Reaper: This Arcane grants healing per kill and a boost to armor, contributing to your overall survivability.
3. [Fury]: more damage on melee.
4. Molt Reconstruct: can give healing each cast of slash dash.

Helminth Options:
Incorporating Helminth abilities can add another layer of customization to your [Excalibur Umbra] build. You can choose to run this build with or without Helminth abilities, tailoring your loadout to your preferences. Here are some recommended Helminth abilities:

1. Nourish: I included Nourish in this build for its energy regeneration boost, which can be invaluable for maintaining your abilities during combat.
2. Ensnare: Ensnare is a great choice for grouping enemies and providing additional crowd control (CC) to your arsenal.
3. Gloom: Gloom offers lifesteal and slows enemies, enhancing your survivability. Keep in mind that it comes with an energy drain, so consider your energy management.
4. Tharos' Strike: Tharos' Strike provides defense stripping and healing, giving you more tactical options during battles.
5. Aqua Blades: more anime.
6. Wrathful Advance: More crit chance bonus
   So on so forth

Statstick and Gameplay:

To make the most of your [Excalibur Umbra] build, incorporating a statstick melee weapon is highly recommended. A statstick is a weapon that you equip solely for its mods and not for its actual attacks. The new Incarnon melee weapons are particularly effective choices.

[Ceramic Dagger] Incarnon:

The [Ceramic Dagger] Incarnon boasts several advantages that synergize well with the [Excalibur Umbra] build. Its Evolution IV provides an increase in critical chance, enhancing your damage potential.

However, I personally use a [Magistar] Incarnon as my preferred statstick. Please note that my [Magistar] Incarnon is not modded optimally and serves primarily as a statstick for my [Khora] build.
EDIT: [Magistar] will probably yield better DMG compared to [Ceramic Dagger]. [Ceramic Dagger] will have better crit chance, and more comfortability with its evolutions by not requiring you to build up combo by meleeing, meaning u can drop [Surging Dash] for something els.

Gameplay Showcase:
For a visual demonstration of the [Excalibur Umbra]'s gameplay, including the prowess of Slash Dash and the build's damage potential, check out the video below:
Video if below dont load

Pls feel free to share your thoughts and feedback.