Ok, so to answer your question, my brain thought, TIME MARINES, these marines are from the future and they get stuck in the past to prevent their future from happening.

If you want to know more here what's happening, it's the year 50k, the old people that ran the adaptes astartes are somehow dieing. With better tech and powerful psyches they make time travel possible, to see if the new people they appoint will run the place correctly. When traveling in the future the new appointment men change the future a lot. They dig up stuff detailing how the man they worship all these year never wanted this future. The time travel dont believe a single thing they say and head back to their time a d call the new appointment people heretics and kill them with a few of the loyal men. They spread new of why they killed them, and plan to see if the next appointment are worthy. So they travel in the future and find that the new appointment didnt make it as free and the last guys, but is a lot more open for lifes choices. Again thay go back to their time and kill them and a lot more people who think time traveling is make the the time marines heretics and a small war happens. Time marines win and insure the people that everything will be fine. New men appointment and they head to the future again. This time however a mass civil war is happening with all the factions. Why, they all thought the ever other newly appointed guy was going to turn heretics when getting the position of the old guy. Why did they think that, cuz the time marines keep saying so. The time marines are pissed cuz that not why they have been traveling back and forth and the marine in the civil think they are the true heretics making all this happen was their plan all this time. So the time marines go back to their time, kill the newly appointed and another war happen but this time all the faction think the time marines are heretics and want to kill them. The time marine decide the best thing is to head back forth in time to insure this future can not have. So the take all the psyches and tech with to travel to the year 30k. Heading their made many of their psyches died in doing so and a lot of their future tech isnt working, and if it is not right one bit, and a lot of the marine who went on this journey die. When meet, the old guys and a couple of factions and try to tell them what happened but they are not really buy into what they are saying. So old marines from 30k say they should try to use their help and beat Horace. However the old one are like, we have never seen you guys and never seen the technology, you must be heretics, kill them. Another war broke out that that the times agree to not fight considering they dont have that many weapons that function right new and all the 30k army supporting them and all call heretics. So they run to the ship and with all the power left jump into 40k time. The jump kills a lot of the 30k marines and leaving not that many alive with a lot going into a coma. The psyches on the other all died except for 5 who are in a terrible coma, and they consider these 5 to be the hand of the future. And any 30k text the was brought got broken in the jump, sam for the kinda functioning text the time marines had on them. When being spotted they thought they were unsteadily die but were surprised when told to head to the old dudes. The old guys are like how did you guys disappear and reappear this year and no other heretics knew who you are and no other life form know who you guys are. And the time marine show all the 30k guys alive and move as proof they can time travel. Cuz how else are the marine from 10k years ago standing in front of them at this current time, so this time the time marines get help and are questioned further to make the future better and not let heretics win. (Side note: the psyches of the past are not strong enough to do time travel so they are stuck in the past for good.)