I've only been playing this deck for a couple days and the list is still pretty rough, but my record with the deck is quite good and I've been having a lot of fun with it so I thought I'd post it now. The deck consists of two interlocking synergy packages, both of which are supported well by Dark Return and Mirror Image.
1) The more obvious (and powerful) package is The Merriest Mandrake + cheap units with relatively cheap ultimates. Once we have two Mandrakes on the board, all our units with ultimates become ridiculously good - even the unassuming Veteran Strategist functions as a 0/4 Evenhanded Golem with summon: gain 5 life - and we can quickly become very far ahead in all respects (board, lifetotal, cards). Dark Return and Mirror Image are crucial here in helping us to keep two Mandrakes on the board (you can still get a solid amount of value with just one Mandrake but two is the real goal, while three is usually more than you really need), as well as allowing us to reuse ultimates.
2) The other package is best described as Dino Killers - we only have 10 dinos in the deck, but that's enough to get value from Avisaur Patriarch's abilities and play the very tempo-efficient Twilight Hunt. Once a dino has been Twilight Hunted, both Dark Return and Mirror Image become very powerful in the traditional Killers way. It's worth noting that Derry gains overwhelm from its ultimate and can be re-ulted each time it is Returned or Imaged, so it really doesn't take much to do lethal with Derry killer attacks. If we don't have a dino for Twilight Hunt, playing it on a 4/4 Lifespeaker can also be good enough (since again, it can use its ult to grow each time you refresh the killer).
In the maindeck, the non-power slots I'm unsure about are Suffocate, Spellshaper, and Twinbrood. You definitely need some amount of removal (and spells for Derry), but it's a little unclear which spells are best, while Twinbrood has nice synergies in the deck but can be a bit clunky. I'm also not sure yet about the powerbase - there are pretty good arguments for various combinations of crests, banners, and seats, and I only started with the all-seat powerbase for simplicity. In the market, all I'm really sure about is that you want Mandrake and Hunt. Azindel has been pretty good for me in the grindier games, but there might be other options.