Muldrotha, the Gravetide is one of the cards recently spoiled for Dominaria. Before I go into any detail, here are his (her?) stats:
Legendary Creature - Elemental Avatar
3UBG
During each of your turns, you may play up to one permanent of each permanent type from your graveyard. (If a card has multiple Permanent types, chose one as you play it.)
6/6
This is a Sultai graveyard commander, and holy shit, is this awesome. Black, Blue and Green are the three most powerful colors in EDH, hands down - ramp, control, card advantage, and plenty of ways to make our commander powerful.
The permanent types we can play are Land, Artifact, Creature, Planeswalker, and Enchantment. That's up to five cards we can play each turn - pretty nutty. Being able to play lands from the grave is very strong, so lands that have sacrifice effects - Strip Mine, Myriad Landscape - are quite powerful - as are landfall effects, such as Lotus Cobra
Artifact removal is common and annoying, but Muldrotha can negate that sort of downside pretty handily. Lotus Petal is basically a free mana each turn until you have something better to reanimate. Ashnod's Altar allows us to sac our creatures to replay them as well.
Creatures that sacrifice themselves to achieve effects are already a staple of most decks - Sakura-Tribe Elder is already in most green decks, but being able to play it over and over again in order to maintain a massive land advantage is dumb.
Bird of Paradise
and Lotus Cobra allow us to grab a massive lead as well in the mana department - Birds and lotus in opening hand will allow you to drop Muldrotha on turn 3, provided you hit your land-drops. Being able to bounce back instants and sorceries with Eternal Witness multiple times is also incredibly powerful, seeing as we cant cast them with Muldrotha.
gitrog monster
fits in here quite nicely as well - being able to sac a land for his upkeep cost and immediately play it is great, and he gives our fetchlands even more value.
Planeswalkers are not the focus of this deck, but are great for forcing your opponents to respond to them in order to draw out hate from our eventual combo. Jace, Vryn's Prodigy
allows us to stack our grave a bit more, and will often transform almost immediately after being played. Garruk Wildspeaker is used as more of a mana-outlet than anything else - untapping two lands each turn is powerful enough, and if he dies, we can simply play him again. Liliana Vess allows us to basically tutor for cards each time we play her, but managing to get her ultimate off is also a nice prospect.
Enchantments that have sacrifice effects are common, but tend not to be played often unless you're running an enchantment-heavy deck. Things like Font of Fertility suddenly become 3 mana land drops each turn, allowing us to play it on our turn then crack it at instant speed, and repeat the process. Seal of Removal is a bounce effect ready whenever we need it, and Grave Bite turns our self-removing creatures into removal. Phyrexian Arena and Rhystic Study are auto-includes into any serious deck that runs U/B, and Sylvan Library is as well.
Now, lets talk Cumulative Upkeep. Mystic Remora, Elephant Grass, Glacial Chasm - cards that are powerful but costly. Most times these cards would be paid for once or twice, then sent to the grave and forgotten once the cost of keeping them outweighed their usefulness - but with Muldrotha we can simply play them again, often without ever touching the Upkeep cost! Illusionary Terrain is in the sideboard to wreak havoc on mono-colored decks (turning all mountains into swamps pisses the hell out of mono-red players). Snowfall is suddenly a permanent high-tide times two - and if you have Prismatic Omen out, suddenly all your basic lands tap for an additional 2 U mana!
Now, lets talk about this deck's obvious weakness - grave hate. Sorry to say it, but most decks tend to run at least one or two examples of it - maybe not quite a Rest in Piece, but Bojuka Bog and Crook of Condemnation are common enough. Your only real defense against this would be counterspells or artifact removal, but we have Elixir of Immortality and Feldon's Cane, which can shuffle it back in at instant speed. You lose your grave, but not permanently, thankfully. If worse comes to worse we always have Riftsweeper to pull back cards we desperately need.
This deck is a tough shell around a volatile combo - Muldrotha is here to provide massive value and allow us to play bits a pieces as needed, but the combo is the standard Deadeye Navigator onto Palinchron/Peregrine Drake into either Memnarch, Labratory Maniac or Blue Sun's Zenith. You can drop Pali and Deadeye with Tooth and Nail, or drop one with Chord of Calling and the other from the grave. Vedalken Orrey allows this to happen at instant speed, so I would suggest either playing that first or making sure you have plenty of counterspells in hand. And, if someone does counter part of it - you can play it again with Muldrotha.
If Deadeye is crucial to this, but if he gets exiled or countered, Palinchron can go off with itself if you have a mana doubler out, such as Mana Reflection, Snowfall and Vorinclex, Voice of Hunger. If, god forbid, Memnarch and Blue Sun's Zenith are exiled, you can mill yourself out with Sidisi, Brood Tyrant and Alatar of Dimentia and win with Labratory Maniac.