Roboute Guilliman is the Primarch of the UltraMarines (one of the more famous Astartes legions in 40k…some say a writer had a real hard-on for them so they got way too much love but here we are).
Primarchs are to Astartes what Astartes are to humans. Where Astartes are super human soldiers Primarchs are even more. Near demi-gods.
There were 20 Primarchs which were scattered by chaos across the galaxy just before they were born. Each landed on a different planet and grew-up in very different circumstances but, being near super human, they all excelled. The Emperor embarked on a mission to find all of them and there are stories about those that read like Greek mythology.
Each Primarch founded their own legion of Astarte warriors and they each had their own character and things they specialized in (all superb warriors though). I think the exceptions are the Grey Knights and the Adeptus Custodes (each more superb than all the others…especially the Adeptus).
After the Horus Heresy (Horus was a Primarch turned to chaos that mortally wounded the Emperor…and then the Emperor nuked Horus) various Primarchs died or disappeared or (from the heresy) many had been turned to Chaos.
Fast forward to present times and there are no Primarchs left (except maybe chaos ones) but their legions still exist.
Long story short Roboute Guilliman has been resurrected and is the only “good” (read not chaos) Primarch currently toddling about.
But he has been gone for thousands of years and the universe is a very different place.
I think GamesWorkshop is, as mentioned above, seeing a need to shake things loose a bit and move along. Sooner or later customers will tire of endless war which is just a repeat of last week’s installment. So Guilliman is back
And the Omnissiah is like it sounds. It is the machine spirit to the Adeptus Mechanicus on Mars and they very, very, very much worship it. The Mechanicus was the one part of the empire that the Emperor forged that was outside of it. The Emperor made a pact with them to leave them be if they built warships and stuff for him and they agreed.
Know that the Emperor was very much NOT religious and wanted a completely secular empire. He was all about that. And pretty much the exact opposite happened after he (kinda) died (he just mostly dead).