Note: This build is NOT specifically for Excalibur (Prime/Umbra), but rather a "universal" build fit to work on virtually any Warframe, provided the Duration, Range, and Strength don't hinder their other abilities.

The purpose of the build is to hit 3 benchmarks with Gloom:
1. Minimal Drain.
2. Maximum Slow.
3. Radius no lower than 25m.

Also note that any empty slots above are considered "flex" for the sake of the build; only the bare minimum needed to complete the prior 3 benchmarks is listed.

Universal Gloom Version 1: Low Duration, High Rewards

This version of the "Universal Gloom" utilizes 190% Efficiency alongside Duration at 40% or higher, and has a comfortable 28m radius. Other requirements for this build that are not listed include:

-Madurai Focus School
We use this for the +40% Ability Strength bonus. Triggering this alongside Molt Vigor takes our Ability Strength to 284%, maximizing the Slow at 95%. The extra Casting Speed helps as well.

-Crimson Archon Shards
You need any combination of these to add at minimum 30% more Duration, so as to pull the number back to over 40%.
OR
-Molt Efficiency
You can wait out the first 6 seconds of the mission for that bonus 36% Duration, which snapshots onto Gloom for a total of 48.5% Duration, enabling the minimal Drain.

Some ideas for things you can use in the flex slots include [Combat Discipline], [Brief Respite], [Primed Sure Footed], [Cunning Drift], augment mods, more Duration, Casting Speed, [Preparation], [Equilibrium], Arcane Energize, Arcane Steadfast, Arcane Avenger, etc.