Disrupt their early game, then play things that don't want to die and eat their life total.

Retribution of the Ancients + Undying & Gavony Township = Removal forever + Unkillable Messengers.

Temple of Aclazotz + Tidehollow Sculler + Athreos = continuous hand eating.

Sideboard needs work, but this is a first draft. Basically I fear GY hate and things in my opponent's hand. Start simple, get more diverse.

Thoughtseize, Inquisition of Kozilek, Path to Exile, Fatal Push... Survive. Also maybe drop a Retribution.

Try to lead with Athreos, obviously game state dependent, but he is a house when your creatures hurt them for EtB triggers. Punish their life total.

When they're at low life Athreos brings all your dead creatures back, just keep playing them until your opponent is dead. Attack! Attack! Attack! Force them to kill your creatures and make decisions, Athreos is probably a creature here as well so bring the 5/4.

I'd really like to add Bitterblossom, and Aether Vial, but they don't really fit. Vial would really be fun because you can respond to board wipes with Athreos.

Even Undying creatures are good with more +1/+1 counters on them, so if your opponent is out of removal then just load up on Gavony damage and start swinging.

There is no sacrifice outlet in the deck other than flipping Arguel's Blood Fast or killing it with Retribution.

If Retribution is turned on then you have more combat tricks than you can imagine:

• 4/3 Messenger dies to -2/-2 because you removed his +1/+1 then comes back for 2 life loss. Gavony, rinse, repeat.
• Use Retribution to dodge Paths, kill your own creatures to force Athreos payments, and potentially get more EtB triggers.
• Gavony + Retribution can reset your Undying while killing things that have taken combat damage already or that are just waiting to be killed.
• Demonlord is a very big flier that has Undying and helps you sacrifice more things to force triggers. Careful, his sacrifice trigger will cause him to exile, not die, if you don't have another creature.
• Erase a Finks -1/-1 with Gavony then unload the access +1/+1s on something that needs to die.
• In response to sweepers, shoot something without Undying or Persist because they'll return before the sweeper and die fully. Shoot an opponent's creature, your Rhino, or something that won't die; additionally, killing an Undying creature leaves it with one +1/+1, so a second shot will not kill one of your Undying.
• Two Demonlords can sacrifice to each other until you run out of black mana to remove the +1/+1s, this will allow you to -X/-X for quite a few things.