THIS is not my updated [Inaros] anymore. I now have 2 Umbral Formas and Gloom etc., and he's over 50k EHP, fast, and very comfortable.
Newest Inaros build is here:
https://overframe.gg/build/213586/

(OLD NOTES)
This is my current 'main'. He's pretty much invulnerable unless you go AFK to get a sandwich while in a Steel Path survival or something.

**I'd also recommend an Aura Forma if you use this frame for lots of different things. I have an Aura Forma on mine, not reflected in this build on Overframe.

I've become spoiled by speed, so after using a different Umbral build for a while, I eventually reworked [Inaros] here to be faster. Makes doing murmur farming and keeping up with speedsters on Sorties much more enjoyable.

I use Magus Repair in my operator, so at the start of every mission, I load up on my Negation Swarm (which makes me immune to Status Procs), then jump into void mode for two seconds to fully heal myself. Without Magus Repair, you could do the same much less efficiently with Devour.

After Helminth, I replaced Sandstorm with [Firewalker], which gives me more speed for 30 seconds when needed, cleanses status effects of myself (if I'm afflicted for some reason. Again, Negation Swarm) and teammates, and provides some crown control. I have another build like this where I replaced my 3 with [Khora]'s Ensnare for doing high level Disruption. I have a third build where I replaced my 3 with a (very basic) Resonator from [Octavia] to help in unique defense cases where I'm alone, like the Void Raider mission. (Replace [Gladiator Aegis] with [Stretch] if using Resonator.) The vast majority of playtime, I use Fire Walker.

This build gives you EHP of 22,457. When Arcane [Guardian] is fully activated, that bumps up to an amazing 41,949. Ridiculous. With the extra speed and status immunity, this basically becomes a very comfortable frame for getting anything done. With Magus Repair and the ability to heal yourself at will without energy--not to mention healing with headshots with Arcane Victory--you're invincible unless you try to die on purpose. Or, maybe, unless you stop to stare at your map for a minute while standing in a Sortie radiation cloud, lol.

• Dessication is for stunning exceptionally tough targets, like Steel Path Noxes. Not needed, but makes for quicker killing.
• Devour is to heal yourself using an enemy. I don't use it anymore because of two other healing abilities on my build: Arcane Victory (headshots), and Magus Repair. If you really need to, you can also use Devour to give yourself some invincibility time while eating an enemy, if you're overwhelmed for some reason.
• Firewalker for more speed when needed and to control areas with fire procs. Also cleanses status effects.
• Scarab Swarm with Negation Swarm augment makes you immune to status effects. Basically to avoid knockdown from Ancients and self-stagger from your explosive weapons.

The only downside of this build is that his powers are all at base level and not very interesting. Some people think that's boring...? This is a build for using weapons and stuff. Kind of a general do-everything setup, and it's really good for that.

If you don't want to use Umbral mods, here's a more basic build without them:
https://overframe.gg/build/95421

If you want to use the Umbral mods but not the Umbral Forma, you can add a Vazarin forma (one of the Gladiator mods) and replace the aura with Steel Charge.

If you want to see my old Inaros Prime build before I made him faster, here it is:
https://overframe.gg/build/76183