At the end game of the Horus Heresy, Horus Lupercal lowered the shields to his flagship and allowed the Emperor, his immediate body guards, Rogal Dorn and Sanguinius teleport aboard. Sanguinius found Horus first. Horus offered him a seat at the right hand of the gods. He refused. And Horus slew him. The Emperor found Horus next, towering over the broken body of Sanguinius. And the rest is known.
This scene is a very, very old one that I recall from the earliest days in 40k. It features models of both Horus and the Emperor (plus poor Sanguinius as well) facing off against each other on board the transformed and warped control deck of Vengeful Spirit. Although its age is really showing and obvious to the trained eye, the diorama is one that is both iconic and unique.