I see some intrinsic flaws in this deck.
#1: Bottomless Pit sucks. Always has, always will. Remove all copies immediately.
#2: The inherent flaw in discard is that it doesn't stop your opponent from topdecking something nice and bashing your face in with it.
#3: Your deck has lots of paths to victory, but doesn't do any of them well.
First of all, the obvious answer: remove all 3 Bottomless Pits. Also, remove both Knights of Stromgald - they're good, but not good enough for this deck and not in theme.
Finally, while Ravenous Rats are cute, they're the lowest class of discard.
You see, I classify all discard into three groups:
Class C - Worst discard. Your opponent chooses and discards.
Class B - Random discard. TSIA.
Class A - Best discard. You choose the card your opponent discards.
What are Ravenous Rats, but Class C discard with a 1/1 speedbump attached?
So, we're left with the guts of your deck:
4x Hypnotic Specter
4x The Rack
2x Megrim
1x Nevinyrral's Disk
4x Hymn to Tourach
4x Duress
2x Stupor
4x Dark Ritual
4x Diabolic Edict
2x Drain Life
Now, I'll argue immediately that you need to categorize them like this:
4x Hymn to Tourach
4x Duress
2x Stupor
4x Hypnotic Specter
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4x Diabolic Edict
2x Drain Life
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4x The Rack
2x Megrim
1x Nevinyrral's Disk
4x Dark Ritual
The first group is your discarders. The second helps you control the board, and the third helps you push the kill through.
Now, let's look at them. Clearly, you don't have enough discard. But what to use?
I'm a big proponent of Addle. By the time you cast it on turn 2, you can often figure out what color your opponent is playing, which makes it a cheaper (albeit slightly more limited) Coercion. Doesn't touch artifacts, though. Let's drop 3 in.
Oh, but we forgot basic land, didn't we? You asked if 18 was enough. I'd say 'no'...it seldom is. Let's try it with 20 land.
4x Hymn to Tourach
4x Duress
2x Stupor
4x Hypnotic Specter
3x Addle
-
4x Diabolic Edict
2x Drain Life
-
4x The Rack
2x Megrim
1x Nevinyrral's Disk
4x Dark Ritual
-
20x Swamp
We're left with 6 empty slots. But wait...we're not done yet.
Relying on your Rack is good, and with this much discard, is likely to be a good course to follow to victory. But what if you don't draw it? You're dead in the water. How about something that helps you draw cards?
Drop in 3 Skeletal Scrying. Yes, I know...but I swear by the card, and you should too. Turns your used discard into card-drawing.
Now for the Drain Life. Fun card? Yes. Good in a deck with very little mana? Definitely not. Out it goes, in favor of more creature control - let's drop in 4 Chainer's Edicts. You now have 12 ways to kill creatures, and one slot left.
Is Megrim good? In the early game, yes. And it'll let you get a few points in. But let's face it...once you're done demolishing your opponent's hand, he's just going to cast whatever he draws, and your Megrim becomes a dead card on the field. Let's lose them both.
Now, for the one I suspect will REALLY raise some eyebrows...lose the Nevinyrral's Disk. (ducks thrown garbage) I know, I know. But what does your kill rely on? Racks. Artifacts. You NEED them to be there, and you can't afford to pop a disk with your treasured Racks in play.
So, that leaves four slots. What do we put there? Well, what's the big problem with our deck? Consistency. We need something that does what The Rack does, so that we have a better chance of drawing into our kill. Well, there's always Demonic Tutor...and I don't see one there. BLASPHEMY! And a little-known artifact called Wheel of Torture can work as Racks #5-7, improving your odds of not even needing to Tutor.
And finally...this one will REALLY turn some heads...lose the Dark Rituals. I know, I know. They're a huge mana burst. And getting one in your opening hand might let you play an extra spell on turn 1. But look at your spells and what they cost. Your discard spells go 2-1-3-3-2. Your creaturekill spells are useless on turn 1. And your other spells can, and sometimes need to, wait. In other words, Ritualing into discard is more likely to burn you than it is to help you...and nothing else is worth the card slot. Not only that, but drawing it in the late game equals a dead card, and nobody wants those.
That leaves us with four slots. What do we put there? Well, it looks to me like you're a little short on discard. I wouldn't mind putting 4 Specter's Wails or 4 Coercions in there. But maybe your environment is more creature-based, in which case you'd want something like Mutilate. I'll leave those 4 slots open for you to metagame with.
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So here's what my version of your deck looks like. What do you think?
4x Hymn to Tourach
4x Duress
2x Stupor
4x Hypnotic Specter
3x Addle
-
4x Diabolic Edict
4x Chainer's Edict
-
4x The Rack
3x Wheel of Torture
1x Demonic Tutor
4x ????? (Metagame)
3x Skeletal Scrying
-
20x Swamp