There are a few typos and its a little hard to read but I'd warrant english isnt your first langauge so thats ok.
On the fluff itself I'd highlight a few things:
now, with only the lost connection to the inquisitor, the imperium nearly has no idear of the threat that lies in the tomb. the thousands of warriors,
controlled by Demen and the high council master control.
I know you used the word "nearly" but its unlikely that the Inquisitor did not send back fairly detailed and thorough accounts of what they found. Even if it was just initial findings. They are known to be very thorough espeically when accompanied by Tech Priests. In addition since it was a Necron Tomb the Imperium would recognise it as a threat even if the info was limited about it.
should Demen ever fall in battle, he will phase out of the material realm, to return to his chamber, to prepare for the next assault, trying to forget his defeat.
Not sure I get this piece 100%. He can phase back to his chamber from anywhere or just the Tomb World?
Where do his minions go? Do they phase out too or?
I'd expand on what motivates/drives Demen, there just isnt enough info on him to get any sort of feeling or level of interest.
Keep at it though, from every seed.....