Archetypes
Out of the two OBR archetypes, Liege is definitely the choice, since Aviarchs aren't Mortisans and Endless Duty is good and fits the messenger ravens sending orders out to the troops.
Liege (6)
M 5" - W 5 - B 10 - Save 4+
Command Ability: Endless Duty
Keywords: Death, Oassiarch Bonereapers, Liege, Hero
Weapons
With nary a weapon anywhere, but a particularly sharp looking beak, I think two sets of Ossified Talons with a twist will do.
Ossified Talons and Beak (1)
R 1" - A 4 - toH 4+ - toW 4+ - Rend -1 - D 1
Enhancements and Abilities
Three abilities stand out right away on this list: Harrowing Shriek, Decapitating Strike, and Spirit Host. Aspected as the Aviarch is toward the avian, and that carrion birds are known for their unsettling calls, Harrowing Shriek feels thematic and gives this backline commander some ability to do damage from afar. As a roguish type, I think he should be able to do critical type damage, and mortal wounds on sixes fits that. And finally, Spirit Host is a great stand in for his messengers ravens to join the fray should he be caught in close combat, if only I had one more destiny point the Spirit Hosts could have four attacks.
Harrowing Shriek (4)
2D6 Bravery test within 9", MW equal to number exceeding Bravery of target.
Decapitating Strike (4)
Model's melee weapons do a MW on 6s to wound.
Messenger Ravens (4)
Talons and Beaks - R 1" - A 2 - toH 5+ - toW 5+ - Rend \ - D 1
Frightful Touch - Rolls of 6 to hit cause MW.
Destiny Points: 19/20
There's a part of me that wants to drop Harrowing Shriek for four more raven attacks. I could see a build with Mortis Praetorians that leans into Harrowing Shriek, but it's probably not worth it in Petrifex Elite, which is what I'd be running the Aviarch in.