Hello there! Can I interest you in a copy of the Codex Astartes, Second Edition? This new...

Space Marines are a bunch of grognards. They think every chapter should need to learn their lessons the hard way, even if it means getting owned super hard and weakening the Imperium's overall position.

Also, the Codex Astartes is a holy tome of a Primarch's wisdom. An actual rewrite would be practically heresy, at least to those that literally hold the Codex sacred. Adding addenda and such for new threats would be far more acceptable.

As a Chaplain of the Razor Wasp Chapter, I'm going to have to pass. As inheritors of both Corax and Khan, we have found that the Codex is neither effective nor efficient and has no place within our tactical doctrine. It encourages wasteful organizational systems and ineffective allocations of material and manpower. I think there's a Fortress-Monastery the next sector over, Black Dragons, I think. Though they're a Salamanders successor, so I don't think you'll have much luck there.

Salamanders were pro-codex once Guilliman told Vulkan he wouldn't have to break his already >1000 marine chapter down further

I thought they were on-the-fence about it.

Both those primarchs supported the codex....

They were, because Vulkan thought he was going to have to break his already shattered legion further

Supported but not fully adopted. And technically, the custom chapter I created is a successor of the Mantis Warriors who received a major gene-seed donation from allies in the Death Spectres to rebuild their losses after they had to detonate their home world. to prevent a Tyranid splinter fleet from gaining the genetic information in the local fauna genome. And neither the Death Spectres nor the Mantis Warriors put much stock in the Codex.

You know, it would be quite interesting if GW actually released a "codex Astartes" and, through some miracle, it actually seemed like something written by a great millitary genuis.
The only tactic I can think of off the top of my head that is for sure in the codex is steel rain. an actual "codex Astartes" would present a lot of different ways to create fluffy lists that employ the tactics that marines use.

Hell, you could even include pseudo-formations that give you benefits for fielding different tactica. however, and I say this as a marine player, stuff like chaos legion rules, IG regiment tactics, craftworld tactics, ork Clan tactics etc. would definitely need happen first, because the last thing we marine players need is more stuff while most everyone else gets nothing

user, it could be a nice story, but I think there's a bit of a problem:
The marlstrom is slightly west of the Galactic Core, or Ultima Segmentum.
The ghoul stars are at the edge of the galacting plane, in the east, IIRC
How can the Death Spectres give enough support to Mantis Warriors, when the 2 chapters are litteraly half a galaxy apart?

>Fortress-Monastery

Black Dragons are fleet-based. The only thing they have about that is an internal legend about a world name Gauntlet that awaits them, and that they would settle upon.
Plus they're only rumoured to be Salamanders successors, so another Chapter would not know that. The out-of-detting knowledge we have is ot shared by the characters most of the time.

MAntis Warriors spent a lot of time travelling around during their penance time, and the Death Spectres do occasionally travel themselves. Most chapters travel around or send out small detachments for different purposes. And the custom chapter was a Mantis Warrior successor, stationed far from where the Mantis Warriors were originally from, which received assistance from a Death Spectre detachment.

Finally we know how to fight Tau & Nids!

Is the OP doing a windows 10 or Fallout 4 thing? I don't mind either, but I just want to clarify

(Mostly Fallout, maybe a dash of Win 10.)

LONG HAVE I WAITED.

youtube.com/watch?v=Cyrg1DaZ9ow

Yes sure (but sarcastically)