Hi
I see that Warhammer: Mark of Chaos and Warhammer: Shadow of the Horned Rat can be bought but Warhammer: Dark Omen which is the sequel to Shadow of the Horned Rat is nowhere to be found.
Any1 know the reason for that? I remember really enjoying that game and would like to play the full collection.

The simple answer is that as a licensed video game series (based on the eponymous wargame), many of the games have been handled by multiple companies over the years. Dark Omen is not handled by the same companies who did Shadow of the Horned Rat or Mark of Chaos.

My guessing is that the rights of that game are under Electronic Arts and/or Games Workshop. One of those examples where nothing is completely clear. Maybe a mess with the game pieces owned by a diferent company. (Art, Lore, Franchise, code, game story...)

Even if the game itself were under Electronic Arts only, the posibility to see it rereleased wouldn't be bright in that regard.

Its available free online, theres a whole forum devoted to it with some mods if I recall, haven't got a link but a quick search should find it.