On a scale of 1 to 10, 10 been CEDH and 1 been worse than a precon this is a 6, it's powerful and can kill players quickly but it is looking to use commander damage which makes things harder.
Greven is a deck that looks to deal lethal commander damage as fast as possible while keeping the engine flowing with the card draw. The best way to do this is through sacrificing creatures with high power and low CMC. Cosmic Larva, Lightning Skelemental and Phyrexian Soulgorger are some of the biggest pay offs for the lowest mana cost. The problem with these is that then come with a downside if they stick around until your next turn, this forces your play patter to be to have Greven ready to attack when you cast them. Lupine Prototype and Ruinous Minotaur can sit around before Greven is combat ready, meaning you don't have to save the mana up if Greven has haste. The next grouping of creatures are the ones that are great to sacrifice but are also good enough to stand on their own two feet as creatures in a deck. Taurean Mauler can grow huge in just one turn cycle and can be threatening in combat as well as to sacrifice to draw cards. Yargle, Glutton of Urborg is similar to the first category of creatures as its big and cheap, however it also has no real downside apart from it having low toughness but with some of the equipment and tricks in the deck it can get into combat easily. lastly in the creature category there is Moltensteel Dragon and Wall of Blood these are the best creature for the deck you can pay life to cast Moltensteel Dragon, increasing Grevin's power if he's out, you can also pay life to increasing both of their powers meaning they are doubly good at increasing greven's power when sacrificing them. Moltensteel Dragon been slightly better as it can get into combat, while Wall of Blood will just discourage opponents from attacking you.
In a deck like this if you don't have creatures out to sac you need to be able to make Greven bigger, the best way to do this is by paying life, the deck is loaded with cards that have value but mean you end up losing lots of life. Doom Whisperer is the best of these as you can pay as much life as you like, easily find a card that you need and kill an opponent with it. Black is also full of ways to draw cards and loss life Bolas's Citadel is one of the best to do this as you can keep casting cards and paying life generating value from the top of your deck in a Future Sight effect.
You also need to be able to clear the way from Greven so he can connect, this deck is full of kill spells among the best are Phyrexian Purge and Fire Covenant that let you pay life to clear the way these can function as wrath effects as well as targeted removal.
The final way to pay life is probably the most powerful, it's K'rrik, Son of Yawgmoth meaning you can pay life to help you cast your spells. He also grows large and has lifelink. This brings me to the next category of how to gain life back after paying it all.
Exsanguinate is a great way to close out the game but can also gain you a bunch of life in a pinch, and Whip of Erebos is also a nice way to give all your creatures permanent lifelink.
You also need to make greven hard to block, and Silent Arbiter and Vorrac Battlehorns means that because of greven having menace he is unblockable while Silent Arbiter also ensures that you don't die on the crack back in combat.
One of the other things you need to be able to do is close out the game Chandra's Ignition means you don't need to deal every opponent 21 commander damage and can kill everyone all at once with regular damage. Tainted Strike, Temur Battle Rage can also kill in one hit for a much lower investment.
The following are some honorable mentions Word of Seizing is a great way to take an opposing creature and sac it acting as removal and just a good value card. Archfiend of Depravity acts similarly to Silent Arbiter as your opponent has to sac down to 2 creatures and if they tapped one of the ones they want to keep they are open for business. Font of Agonies means that all your life you pay acts as removal. Finally there is Vilis, Broker of Blood meaning you draw a bunch of cards from doing everything the deck wants particularly with K'rrik, Son of Yawgmoth in play.
