Quote Originally Posted by blackjack217 View Post
Specifically the terminust est, a death guard vessel. This doesn't stop Dorn from being killed on the sword of sacrilege, which is specifically mentioned to be a despoiler class battleship despite the fact that the design (which is distinctly different from the Terminust est) will not be made for another 5k years. Of course this is almost certainly GW not reading their own fluff, but its just more fun to assume its some sort of conspiracy.
Alternatively, it was a design lost and rediscovered, hence the emphasis placed on your 'based on an older design'