The premise...an Ultramarine Captain that looked both savage and as noble as only a son of Macragge can be.
It wasn't going to be easy. Ultramarines are pretty liberal as far as decorating their armour with skulls, so that wasn't the way.
But what about real skulls? All Chapters use them, but Ultramarines and their most faithful successors don't exactly think it's awesome to go around with a skull on themselves, they denounced the World Eaters for headtaking, during the Great Crusade. So there's a solution, but one-trick ponies don't cut it.
I could go for an Mk5 studded armour, but we've all seen hundreds of Ultramarines wearing those. They're awesome, but I wanted a real challenge, an UM captain should go around in richly detailed armour, with plenty of eagles and such. Also, using Low Gothic (English, for us viewers) instead of High Gothic (Latin) also makes Grim more "down-to-earth".
So enough of wearing bumper stickers stating how much of a bad boy you are. Octavius would have to show it by doing what Space Marines do best. And for all your doctrine-shattering purposes, there's no better tool than a chain-axe, available in every Frothing Sickos R'Us. And why not take two? As long as they're all decorated with inverted Omegas and eagles, everything should be fine...
The wolf-pelt cloak and the left shoulder pad (an adapted Dreadnought leg armour), as well as the crest, weren't my ideas.