I was inspired by harrisonium's Scarecrow Scramble, a super-budget, tron, scarecrow tribal. This deck follows that approach, but it is more budget than super-budget. You can achieve super-budget status if you replace the copies of Metallic Mimic with Bloodline Pretender.

Strategy

Assemble your Urza lands to cast Reaper King paired with copious amounts of scarecrows, destroying your opponent's creatures, artifacts, lands, &c.

Jack-o'-Lantern is a turn one play to draw a card and even hate on someone's graveyard. Expedition Map is better, but things cannot always go your way. Jack-o'-Lantern is pretty flavorful anyway.

With luck, by turn three or four, you have assembled the lands to beckon your scarecrow army, but Reaper King is still out of sight. The mana provided by Scuttlemutt or discarded Jack-o'-Lanterns can get him out as early as turn four, but expect him later. Meanwhile, develop your board with Metallic Mimic (or Bloodline Pretender) and get some nifty artifacts in play.

Mimic Vat is an interesting card. Once a creature dies, you can imprint its image on the card to spawn a copy with haste for three mana each turn. This goes great with Reaper King, Scarecrone, or Artisan of Kozilek.

Scarecrone is a center point as well. Though sacrificing scarecrows for cards is questionable, the option is still there. Her second ability can bring back Reaper King back from the grave.

We need scarecrows to activate the Reaper King's destructive ability. Scorn Effigy can be foretold to trigger it the very turn you summon Reaper King. Wingrattle Scarecrow, Watchwing Scarecrow, and Rattleblaze Scarecrow are somewhat mana-expensive, but they get some cool keywords for having creatures of many colors. Scuttlemutt provides that source of color despite being colorless. Note that Wingrattle Scarecrow and Rattleblaze Scarecrow can gain Persist, which triggers Reaper King's reaping.

Sideboard

Try Heap Doll for cheap scarecrows for Reaper King triggers.
For removal before Reaper King, try Spatial Contortion.
All is Dust crushes hopes and dreams.