Here's an update on a deck I've written about on The Command Zone before. It was originally designed around Liliana's Caress when that card first saw print. "Megrim" is a classic archetype, and this is my personal favorite incarnation of it to date.
This build caters to Multiplayer play quite a bit, but I don't feel it sacrifices much of its 1v1 power, because a lot of the symmetrical effects don't necessarily get weaker in 1v1. Syphon Mind is the one card that just plain sucks in 1v1, but it's so good with 3 or more opponents I had to include one.
4x Liliana's Specter
2x Bloodcheif Ascension
2x Innocent Blood
4x Liliana's Caress
2x Sign in Blood
3x Liliana of the Veil
3x Underworld Dreams
1x Syphon Mind
4x Burning Inquiry
1x Rakdos's Return
2x Howling Mine
2x Anvil of Bogardan
3x Graven Cairns
1x Blackcleave Cliffs
4x Dragonskull Summit
2x Blood Crypt
2x Rix Maadi, Dungeon Palace
Let's start with the creatures. Liliana's Specter is the only one that made the cut, clearly. I've also run Rotting Rats in the past and found that to be... acceptable, but not strictly necessary. For strictly 1v1 play, I'd consider Ravenous Rats as well, but that card loses a ton of it's value in Multiplayer. Augur of Skulls is also potent and on-theme, but again I'd prefer that card for 1v1 but not so much for four-player games.
Moving on the spells, we have three main components: Draw, Discard, Punishment. The single most pressing change I need to make to this deck is increasing the Anvil of Bogardan count to a full 4x. That card is just SO good in this deck.
A quick aside about Anvil of Bogardan - in this deck, that card also serves as a wonderful, built-in fine-tuning helper. If you were to put 4x Anvils in the deck, play 100 or so games against a variety of decks, and record every single card you discard to the Anvil over those games, you'd probably have some pretty good data on what is and isn't critical in your deck. I haven't applied nearly so rigorous a scientific approach, and I haven't played nearly so many games, but I am already finding that I end up discarding Underworld Dreams more than any other card, save extra lands when I have too many of those. Bloodchief Ascension is probably #2 on the "most likely to be discarded" list. The point is, just pay close attention to what you're discarding and how often, and that should give you some valuable data for fine-tuning the deck, if you feel that's important.
A lot of games with this deck can be fairly non-interactive, to be honest. Most of the time, you just try to ignore whatever it is your opponent is doing, stick a couple of key Enchantments, and then unleash a salvo of Blightnings, Burning Inquiries, and hope that gets you there. That often is enough to get you there, but when it doesn't, Innocent Blood, Liliana of the Veil, her Specter (as a blocker), and Rakdos's Return all help provide some staying power. Syphon Mind and Sign in Blood help provide a bit more gas if you run out of steam.
There are a number of small changes I'd likely make if I weren't trying to straddle the gap between 1v1 and Multiplayer, but as I said earlier, I don't think I lose much power at all by straddling that gap, so it's not much of an issue for me.
One final note toward strategy: Don't be afraid to cast a Burning Inquiry on Turn 1 if you don't have any Megrims or Caresses in hand. If you have Caress or Megrim in you opening 7, though, I would definitely try to stick those before you use your Inquiry. But, messing with your opponent's opening hand like that is hilarious and surprisingly effective. I've earned "Turn zero scoops" before because my opponent kept a slightly dodgy hand and my Burning Inquiry on the play suddenly left him with 7 non-land cards to start. It's also hilarious when your opponent makes a random, arrogant comment about what a "snap keep" his hand was... "Mountain, Inquiry, fuck your God-Hand, go?" Of course it's random, so occasionally you'll actually improve their hand - but it's worth the risk for the times it just griefs the hell out of them.
Anyway, it's a fun deck to play, and great for very quick games, but it's pretty non-interactive, and by some groups' standards, totally anti-social. I still love it, though.