Wouldn't you know it, the hardest level in one of the hardest games ever made is the mirror equivalent to Sheol, Hades, Heck, the Underworld... whatever you prefer. Sure, Dark Souls has plenty of areas where getting stuck for weeks on end is an inevitability... Blighttown, Sen's Fortress, the crystal cave before Seath's boss fight, they're all infamous in their own right, but they don't hold a candle to the melting inferno that is Lost Izalith.
Lost Izalith is accessible after you surmount Blighttown and defeat Quelaag the Chaos Witch, which is roughly about a quarter of the way through the game. You won't bother going into it until well past that point if you wish to keep your sanity, however, because you have to defeat the Ceaseless Discharge to even gain access to most of the area, and with a name like that, you KNOW it means business. It's one of the dodgiest and cheap bosses you'll encounter if you fightit the normal way, but it can be lured to a spot where it cannot move or attack, and is easily dispatched by hacking at its arm, knocking it into a pit and killing it instantly.
Unfortunately, this is where the level's mercy runs dry. Right after this, if you intend to go any further, you have to brute force your way through several minotaur demons, a battalion of CLONES OF A BOSS that you fought EARLIER IN THE GAME (sometimes TWO AT A TIME), gigantic centipede monsters that can degrade your equipment, and all sorts of nasty traps and miscellaneous hazards, all of which are easily capable of slicing off your life bar in one or two good hits.
This isn't even delving into the boss fights, which are some of the most ludicrously taxing and soul-crushing fights in the ENTIRE SERIES. And there's four of them! FOUR! In pretty much every other area of the game, you may have to fight, say, two at maximum.
There's the Demon Firesage, whose name is a lie as its attacks aren't even fire-based, pulling a bait-and-switch on players who stack fire resistance for the fight. Then you have to contend with the Centipede Demon to gain access to a ring that allows you to traverse over the lava, and you have literally two small islands of rock to fight the damn thing on, as the rest of his arena is covered in magma. Not to mention he can lay you out with maybe two attacks tops. At the culmination of your cumbersome trip to Hell is the Bed of Chaos, easily the cheapest and most frustrating fight in the game, as it's not even clear how you're supposed to HURT the damn thing and it gains so many stupid arena-spanning attacks that it practically becomes a luck-based fight.
So there you have it, if you're looking for the creme de la creme of cheapness and unholy difficulty in Dark Souls, look no further than Lordran's own personal underworld. You'll link the fire, alright... when you burn in this place for damn near an eternity.