I have a few quick suggestions to make this deck go faster. Slumbering Dragon, Adaptive Automaton, Dragonspeaker Shaman, Dragonlord's Servant, Eternal Dragon

Thanks for the suggestions! Slumbering Dragon looks like a good option for stopping fast aggro. I'm going to try to find space for Dragonspeaker Shaman and/or Dragonlord's Servant. I'm not sure that Adaptive Automaton and Eternal Dragon actually make this deck faster. Adaptive Automaton isn't fetchable with Scion of the Ur-Dragon and I've already got better "lord" effects in Dromoka, the Eternal and Bladewing the Risen. Eternal Dragon just seems bad in a reanimator deck. A 5/5 flyer for is pretty lackluster.

This would be great but once you activate scion once it is no longer scion and so the second activation would only put a dragon in your graveyard

Can you explain the combo with Bladewing the Risen + Shapesharer + Scourge of Valka? How does this work exactly?

DonaldFuck So you have Shapesharer in play, then you cast bladewing and let him enter. With his etb on the stack you activate shapesharer copying bladewing. Legend rule happens and you sac original bladewing. Then with bladewing trigger on the stack you select to bring back bladewing you just sent to the graveyard. Then his etb triggers again and the legend rule happens again. Rinse and repeat for etb triggers. Meanwhile each time bladewing enters he triggers scourge of valakas, dealing at least 2 damage each time. Hence it kills all your opponents

The Shapesharer combo does not work, because the etb trigger of Bladewing has to target something as the trigger is placed on the stack. So you cannot target Bladewing himself because when the ability is triggered, Bladewing is not in the graveyard.

Rule 114.1. Some spells and abilities require their controller to choose one or more targets for them. ... These targets are declared as part of the process of putting the spell or ability on the stack.

But there is another way to do this combo: you have dragon tempest or scourge of valkas on the battlefield with Scion of the Ur-Dragon. You activate Scion's ability to copy Bladewing and put him into your graveyard. You then reanimate him with animate dead or any other reanimation spell. When Bladewing enters the battlefield, it dies immediately because of the legend rule, then the triggers go on the stack: you deal some damage to someone, then you recur Bladewing back and do this again and again.

The difference this time is that the legend rule applies before the triggers go on the stack, and this time the loop works.