This deck is build to be a versatile deck able to adapt to many situations while still playing a ton of Zombie cards.
Win conditions:
21 General Damage with Grimgrin, Corpse-Born. This often the easiest way to win as by default it only takes three hits to KO an opponent. Cards like Batterskull and Savor the Moment help to speed things up.
Massive zombie beatdown. This deck plays alot of Zombies and alot of Zombies are quite good. With cards like Grave Titan and Army of the Damned it is easy to build an a legion of Zombies to quickly end the game.
Be lame and combo off. With Rooftop Storm and Gravecrawler or Risen Executor you are able to make Grimgrin arbitrary large. Combined with Diregraf Captain you are able to kill off the board and Diregraf Colossus makes alot of Zombies. This my least preferred way to win but sometimes after 4 hours of playing you just want the game to end. :)
With so many different ways to win and a lot of ways to bring out the tools that you need to win this deck when played right is quite powerful.
Its greatest weakness is that it is slow to get going, has no creatures with flying and can be weak vs strong consistent control or graveyard hate.
Since most of the deck is pretty straight forward I will talk about some of the cards in the deck and the synergies.
Ways to Untap Grimgrin, Corpse-Born:
One thing to point out is how to even use your Commander by untapping it in the first place. The easiest way to do this is with these cheap creatures (Gravecrawler, Reassembling Skeleton, Bloodghast, and Relentless Dead) that have built in ways to bring them back to battlefield.
There are also a number of other cards (Lich Lord of Unx, Westvale Abbey
, Diregraf Colossus, Grave Titan, and Batterskull) that produce Token creatures that are convenient to sacrifice.
Lastly there are two cards that simply let you untap Grimgrin which are Fatestitcher and Minamo, School at Water's Edge
Card advantage engines:
Skullclamp, Erebos, God of the Dead, Phyrexian Arena,
Kindred Discovery
, and Cryptbreaker all provide great ways to cards. Crucible of Worlds and Liliana of the Dark Realms also provide card advantage through lands.
Notes about specific cards:
Rooftop Storm can be used with casting Gravecrawler, or Risen Executioner for free from the graveyard. It does not matter how many creatures are in the graveyard for Risen Executioner since you can always choose to just pay 0 for the cost. Rooftop Storm also lets you cast a Zombie from the graveyard for free per turn with Gisa and Geralf or 1 colorless with Havengul Lich (from any graveyard).
Food for Grimgrin (Gravecrawler, Reassembling Skeleton, Bloodghast, and Relentless Dead). These creatures are the primary way to untap Grimgrin as noted above but there are some important things to keep in mind. It is often unwise to play a second version of one these cards as they are very important to untap Grimgrin effectively and are often on people's radar to exile. Of the above cards Gravecrawler is the best as playing one black mana to untap Grimgrin is such a great deal. Reassembling Skeleton is not far behind Gravecrawler as you have to pay an extra mana but you have a card that be returned at instant speed avoiding many graveyard hate cards if played carefully. Bloodghast is often a reliable way to untap it does require playing a land which can be difficult later in the game but the fact that it is free can be just what you need to pull some crazy things. Lastly Relentless Dead is the most expensive one to untap Grimgrin but late game can be very powerful as it allows you to put a Zombie from the graveyard to the battlefield as long as you have the mana.
Karn Liberated and Spine of Ish Sah are important cards as they are the few ways to deal with things like enchantments. Be careful about using these cards for anything other than must be removed cards.
Thassa, God of the Sea this card is a cheap way to provide a way from Grimgrin to be unblockable as often you will have an opponent with an indestructible creature or several blockers. Also scrying 1 every turn is not half bad as well.
Tolaria West is a great card in this deck with all of the utility lands that it plays. The only downside is that it is often hard to have double blue but the benefit to having a quasi Demonic Tutor is more than worth it. You can search for a way to untap Grimgrin (Minamo, School at Water's Edge or Westvale Abbey
), evasion for Grimgrin (Rogue's Passage), or a way to deal with hexproof creatures (Arcane Lighthouse), ways to recur creatures from the graveyard (Volrath's Stronghold or Unholy Grotto), sacrifice outlets (High Market or Phyrexian Tower), a way to give yourself no maximum hand size (Reliquary Tower), graveyard hate (Bojuka Bog), some ramp (Myriad Landscape) or just the missing part to the Cabal Coffers / Urborg, Tomb of Yawgmoth combo.
Phyrexian Tower and High Market are great cards in this deck as they provide an extra layer of protection for your preferred food for Grimgrin as Grimgrin often will die and it will expose your other creatures to exile effects. Phyrexian Tower can also function as a ramp with Bloodghast easily as well.
Crucible of Worlds may seem like a strange inclusion but it plays an important role of providing protection from land destruction and also with the aid of fetch lands to provide card advantage by allowing constant land plays. It also works well with Strip Mine and Wasteland to cut your opponent off troublesome lands such as Maze of Ith. With Myriad Landscape and a few turns all the basic swamps in your deck will quickly be placed in play. Tolaria West, Fetid Pools also give you a bit of extra value.
Some notes about the deck:
There are a total of 12 swamps in the deck 4 of which are also islands (Underground Sea, Watery Grave, Sunken Hollow and Fetid Pools.
I wanted to make it close to Mono Black as possible and also make it Zombie theme as much as possible as well.
This deck is designed to be played with in a 4 player game. This is why having something like Strip Mine and Crucible of Worlds in the same deck is not a big deal and to a lesser extend the combo of Gravecrawler and Rooftop Storm. You may want to adjust some cards if you are playing in smaller/larger games.