First, this basic concept is from kristhestoner's "Maralen the meme" deck on here, then I made improvements and added a lot to make it faster and more consistent... now it's a proper cEDH deck that can win on T1 if you draw a really lucky hand, and T2 wins shouldn't be unheard of either with a little luck, with T5 wins being the baseline expectation, assuming you draw no ramp to start, or the first couple turns.

So this is a fun/silly deck, but it's a game-winner as soon as you can cast Ad Nauseam, which will be T5 at the latest, assuming you get to search for it before Maralen of the Mornsong is removed from play, or as stated before you can win T1 too (draw Ad Nauseam or Demonic Tutor and enough 0 cost ramp).  Even if you have to tap out to cast Ad Nauseam, you will eventually "draw" into 0cc mana sources that produce black mana (Moxes, etc), which will allow you to cast Dark Ritual to keep going into the combo.  Darl Ritual and Rain of Filth ensure you will have enough black mana to cast Thrull Parasite and Sickening Dreams, and then pay for Thrull Parasite's extorts to give you the life advantage to win after discarding all the lands and 0cc spells you don't need, dealing lethal damage to each opponent with Sickening Dreams.  Thrull Parasite may have to be used 4+ times to recoup from Ad Nauseams' life loss from cards that cost 1-2cc, so it should be the first card you cast once you have 2+ mana available, with plans to find and use other mana sources to make up that life.  It's a silly deck, but effective.  And also not a deck your opponents will probably let you play more than once, or they will know to play control decks with counterspells ready to stop you, since they will be able to search for them on their first search from Maralen, before you get to use her.  Ha!  Have fun!

The deck is full of 0cc/free spells that won't hurt you with Ad Nauseam, but may help you either accelerate your wincon (mana rocks/spells), or survive attack damage to limit your need to use extort later.

In the event you get lucky enough to draw Dark Ritual or Jeweled Lotus in your opening hand, allowing you to cast Maralen T1, search for Mana Crypt/Sol Ring/Mana Vault T2, then T3 Ad Nauseam and you're off to the races with your third land drop and your mana rock allowing you to cast it.  If you draw other usable mana sources in your opening hand on top of Dark Ritual/Jeweled Lotus, you may be able to pop off on T2, or even T1 if you draw Ad Nauseam in your opening hand with a Dark Ritual and/or enough mana rocks.

Regarding making up a life deficit to opponents with Thrull Parasite and extort, there are nine mana ramp sources that will net you anywhere from 14-17 extra mana from the use of just one mana (Rain of Filth will depend on how many land drops you've made, with T2-T5 equaling 2-5 mana), with 8-12 of that mana being black.  Say you draw out 90% of the deck... at most you would be pinged for 15 life from Ad Nauseam for the cost of all your 1 and 2cc spells.  If you can cast and use all the ramp in the deck, you can be +8 to +10 black mana and +5 colorless mana, giving you plenty of breathing room to pay for the ~5 extorts you'll need to make up the lost life from Ad Naus.  You'll probably need to draw out at least half your deck anyway to be able to pay for everything and then have enough cards to discard to Sickening Dreams (probably need to deal ~37 damage to each opponent, and therefore discard ~37 cards).  All depends on the shuffle.

Put Arcane Signet in the deck if you don't have any other mana rocks that can tap for black, like the Moxes.  Then a Sol Ring will let you cast Arcane Signet, allowing you to cast Dark Ritual or Rain of Filth, and then you can go to town.

Chrome Mox is likely only usable after drawing a lot of cards with Ad Nauseam, as there are only five black nonramp cards that you won't want to use after casting Ad Nauseam (i.e. the tutors, and Slaughter Pact).  But being able to use Chrome Mox after Ad Nauseam is cast is the main reason it's in there, so dig deep if you have to.

Initiates of the Ebon Hand is not exactly a necessary card to include, but it could make your Ad Nauseam draw more efficient, turning colorless mana into more usable black mana, potentially requiring fewer drawn cards overall to extort enough and still get your lethal damage in.  But you're using one black mana to cast it, so you need to have a need for 2+ black mana (without current access to it), and have 2+ colorless available. 

Defense Grid is your safety net against at least one counterspell, if not more if they let it enter play, at which point even free-to-cast counterspells cost 3cc.  Welding Jar protects your Defense Grid from instant speed removal once it's in play, so cast Welding Jar first.

Codex Shredder lets you (eventually) retrieve Ad Nauseam, should it get countered.  Vampiric Tutor, Imperial Seal, Demonic Tutor, and Profane Tutor let you find Codex Shredder sooner in such a scenario.

Chalice of the Void is on the Maybeboard, because with all your important cards costing 1cc and 2cc, and Maralen at 3cc, it would only be safe to cast at X=4, so... it's probably not worth putting in, unless you have it laying around and think you'll find yourself with eight available mana at some point, and a need for it.  I just don't think that should really ever be the case, since you should be winning before then.

Scroll Rack got a recent add.  The benefit is pretty big, as once you draw it with Ad Naus it will give you a more efficient path to the win, but the real hope is you draw it in your opening hand and can both cast and activate it ASAP to swap lands for more ramp in the following cards, potentially securing a T2 win, pending how your opponents respond to everything.