Personal [Oberon Prime] build that can do it all. The Helminth ability I choose is Nourish for the Viral Damage multiplier buff and Arcane Energize effect. I have a complete armor strip on reckoning for Steel Path. I become Immortal through the use of [Adaptation], [Hunter Adrenaline], and [Quick Thinking]. I am an immortal support that gives out big healing, armor, and damage buffs for a long duration. Hallowed Ground's Radiation and Reckoning's Blind serve as great crowd control tools that make endurance runs easy.

3-Step Guide to True Immortality:

• Step 1: Use Growing Power for additional Strength.
• Step 2: Use Nourish, get your Health damaged so you regen all your energy to max.
• Step 3: Cast Hallowed Ground + Renewal. (Renewal will restore your entire Health on Initial healing).
• Step 4: You are immortal already thanks to [Quick Thinking], keep using all of your abilities. The Panic button is Reckoning for a full Armor Strip, Shield Gating, and possible Health/Energy orbs. If you're low on Health recast Renewal for a full healing after disrupting enemies with Reckoning.

Explanation: Any health you gain can be converted to Energy through Hunter Adrenaline. Nourish multiplies energy gain by 1.9x/3x and produces Viral stacks when you attack or are attacked. Additionally, Renewal restores health, which is what is needed for Hunter's Adrenaline to function. Any abilities you use will feed this infinite energy/health loop making you immortal through [Quick Thinking]. If you don't have Molt Augmented, then exchange [Overextended] for Hollowed Reckoning with [Cunning Drift] and use Arcane Avenger, or whatever damage arcane you enjoy. However, Arcane [Guardian] is critical. It's best not to replace it, but if you don't have it then Molt Reconstruct, Arcane Blessing, Arcane Pulse, Arcane Reaper, Arcane Tanker and Arcane Grace are all fine choices.

Stuff to note 📝:

• For Eidolon Hunting, change [Growing Power] into [Dead Eye], and remove Nourish so you can equip [Smite Infusion] instead of Hollowed Reckoning. [Xata]'s whisper is good too, but a bit of an overkill. For Energy during Renewal, you can exchange Molt Augmented for Arcane Energize, and get stepped on by the Eidolon to regen Energy through Hunter Adrenaline.
• For Disruption missions, you can do very well with a full armor strip, the key is to use Hollowed Ground and then Reckoning. Nourish gives Viral damage which is good versus red health, couple that with a Slash weapon like Felarx and it becomes an instant kill.
• In regular star chart, I think you could swap [Hunter Adrenaline] or [Adaptation] for [Equilibrium] since Reckoning spawns health orbs if you find you aren't taking enough damage to keep energy up. Works in Onslaught anyway while ranking. -User40921
• Equipping the Decaying Dragon key, makes damage to your health easier. Using two augur mods into your sidearm means you can use Shield Gating too (Reckoning for full shields).
• The focus school I use is Unairu for Steel Path and Madurai for Eidolon Hunting. Zenurik works too outside of Renewal and Vazarin can be good for its sling. Naramon is quite fun too if you are melee-centric.
• Use [Primed Sure Footed] if you have explosion weapons, it will cost you one additional Forma. However, you shouldn't need it if you are actively using Hollowed Ground and Renewal.
• Instead of [Blind Rage] you can use [Transient Fortitude] or [Archon Intensify] for better energy economy. Reckoning needs 200% strength for a full armor strip which can be naturally achieved through Molt Augmented. I use [Blind Rage] to offset [Overextended] strenght loss and because Renewal with ~100 HP [Regen] is the sweet spot for becoming immortal through [Quick Thinking]. Also, the higher strength aids in keeping my teammates alive with the armor bonus from Hollowed Grounds and Hollowed Reckoning if you use the augment mod.
• Using Renewal on top of Hollow Ground gives Iron Renewal buff, which is just bonus armor, stagger, and status resistance. It can affect allies too, if they stand on it at the time and some time after they leave.
• Hollowed Reckoning can be swapped in instead of [Overextended] for its 40% range and armor bonus with no STR loss but you'll need to exchange Power drift with Cunning Drift to obtain a comfortable range. I mostly do that to free up Molt Augmented for something else.
• [Phoenix Renewal] can be slotted in place of Overxtended for endurance runs or Archon Hunts. This build only makes Oberon Immortal, but your squadmates remain mortal.
• I am using both [Stretch] and [Overextended] to bring the Range of Reckoning and the radius of Hollowed Ground to a comfortable level. You could also use [Augur Reach], [Cunning Drift], or Hollowed Reckoning instead. I enjoy using Hollowed Reckoning for the armor bonus and because it provides a +40% Range to Reckoning so it's better than Phoenix Renewal in my opinion.
• [Primed Flow], [Archon Flow], or [Flow] is necessary for this build because we are tanking on energy and for casting abilities often despite Blind Rage's negative efficiency.
• Despite using [Blind Rage], there should be no energy troubles if you are taking damage, and you should be able to spam reckoning if the enemies are of a sufficient level. If you are in low-level Steel Path it might be better to use an unranked [Adaptation] instead of a ranked one. If you are in the regular star chart then just use anything you like instead of Adaptation because the additional durability might do more harm than good, anyway, you shouldn't have the mod unless you trade.
• Molt Augmented can be replaced with an Arcane Avenger or an Arcane damage of your choice so long as you achieve the 200% STR nedded for 100% Armor Strip. However, Arcane [Guardian] is absolutely necessary if you are doing endurance runs on Steel Path, outside of SP you can replace Arcane Guardian with any other arcane.
• For regular Archon shards, the only ones l would suggest are 2 Amber Casting Speed shards. Then 3 Ability Strengths shards because it enhances all of [Oberon]'s abilities.

Good luck and have fun Tenno :)