Sacrifice after IV.) I, II, III Exile cards from the top of your library until you exile a legendary card
Once your Humans are all in play, you can clear the battlefield with Taunt from the Ramparts, goading all your opponents creatures and preventing them from being able to block, opening the way for your creatures to sweep through
Thats usually how Triumph of the Hordes plays
Aetherflux Reservoir A lot of Commander decks run Aetherflux Reservoir as a method to gain life and a way to eliminate other players
This is good in an aggressive deck, and I think black/red will be especially interested in this, just make sure youre likely to maximize both cards as much as possible
The second Greyjoy legend I selected is one of my favorite characters from the show, Yara