This deck is not very good.

It's an aggro deck, in that if you don't win by turn 5 or so, you will lose. It is also a Timmy deck. It can make creatures that are quite large, up to 41/41 creatures. And these big creatures cost 2 mana. And you can play up to 12 of these creatures in your deck. And you can give them trample. What's the catch? The catch is that it is all dependent on your graveyard.

The big creatures are Fiend Artisan, Urborg Lhurgoyf, and Old Stickfingers (the undergrowth fatties), all of which have some variation of the text "This creature gets +1/+1 for each creature in your graveyard." The trick, then, is putting lots of creatures in the graveyard very quickly. This can be accomplished by milling yourself with cards like Stitcher's Supplier and Urborg Lhurgoyf itself, and with 42 creatures in the deck, those mills will be very effective. You can get away with playing 42 creatures by playing 17 lands. And you can get away with playing 17 lands by playing 7 Llanowar Elves and filling the rest of the deck out with cycling creatures. The cycling creatures both serve the purpose of being able to be thrown from the hand into the graveyard to give extra power to your undergrowth fatties and moving through your deck to find the requisite number of lands. Additionally, some of the cycling creatures are Titanoth Rex, with which you can give your big creatures trample and make it so you don't get chump blocked forever.

The 1 non-creature in this deck is The Great Henge. You can make it cost 2 mana with the big creatures, which makes it essentially free. And it's a good card to be essentially free. The question you might have is "is it worth it to play 1 non-creature when cutting it lets you play Umori as a companion?" The answer is that I've tried Umori companion, and it is not worth it. The main reason is that if you ever spend the mana to cast Umori (3 to buy, 4 to play), the game has gone late enough that your opponent is probably in such a winning position that the Umori is completely irrelevant. The other reason is that almost none of the creatures can get a cost reduction from Umori, so it isn't worth it. The other card I considered was Call of the Death-Dweller, which seems good on paper, but has equal probability to be milled and isn't always not awkward in hand. The new card from Woe: Cruel Somnophage seems a good addition too.

I made this deck best of 1, mostly for the reason that your opponents can sideboard in graveyard hate much more easily than you can sideboard hate against them. If you really want to try, you could do Haywire Mite for artifact/enchantment based graveyard hate, Kitesail Freebooter to try to rip the hate out of hands, hexproof effects for you, like Surge of Salvation, Keen-Eared Sentry, or Metropolis Reformer, but those all require white.

For going to Pioneer, there's cards like Strength of the Fallen or Life's Legacy, but they're not really much better than options already in Explorer.

Anyway, in the youtube video, I also talk about probabilities and whether 17 lands is really enough (it is) and how many cards you can expect to cycle with a certain number of cyclers in your deck.