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Last time we got punted into outer space or something, and found a portal to Hell. Now we're going to go through Hell and find what's at the bottom of it so we can kill it. Because secretly, what ToME always wanted to be was a Doom fanfiction.
The Nether Realm starts at level 666, and we have 30 levels to get through. All of the monsters are leveled up, but fortunately, their levels seem to cap out at 150; by comparison, in the Void we were seeing monsters at level 130, so 150 shouldn't be too much worse. Naturally, it keeps the Void's gimmicks of forgetting the map as you go and not having any air. Additionally, it periodically hits you for about 30 nether damage, which isn't that much, but it really puts a damper on using resting to recover health (so we have to use spells instead). Finally, the floor is now composed entirely of nether mist and lava, which hit you for nether damage and fire damage respectively. That's honestly not too bad though (not least because we're immune to fire); if nothing else, we can create normal floor tiles by casting Time Out and then tunneling.
We spawned next to no fewer than three down staircases; the game isn't going to be that generous in future.
And here's where I regret not bringing the Ring of Flare along; there are also dark chasms (the gray "#"s), which can only be crossed by flying, and we can't currently fly. Fortunately they're unlikely to get in the way much.
The Nether Realm has an awful lot of vaults and monsters.
They also seem to wake up unpleasantly quickly. We can still dodge most fights by throwing up gigantic globules of granite, but not all of them -- and the monsters around here are unpleasantly durable, and unsettlingly good at resisting the slowing effect of Black Hole. Our offense is still unreasonably strong, for as long as our SP holds out...
While working through the vault from the previous screenshot (in hopes of finding a staircase in it, naturally), we run into the leader of the Ringwraiths, the Witch-King of Angmar:
(Shown here as a gray "W", having just summoned some chump minions)
He's level 150, like everything else around here, and is thus as strong as he possibly can be. It takes us several rounds to slow him to the point where he's not getting turns any more; in that time he manages the above summon, a few movement actions, and a round of melee in which he tags us, again, with the Black Breath. But that's all he gets.
The Witch-King of Angmar says: 'The fat lady sang.' The Witch-King of Angmar is destroyed.
And with Sauron dead, he's gone for good.
Moving down a trapped corridor, we step on a tile that looks clear, and
You set off the unknown trap! You are hit by nether forces! *** LOW HITPOINT WARNING! *** You identified the trap as Nether Bolt Trap.
And like that, we went from having over 1200 HP to having 65. If I'd been marginally more sloppy with Mitch's HP total (our max is 1502), the LP would be over right now. Traps in ToME do not fuck around, and they do not obey the damage caps -- nether is supposed to cap out at 550.
I guess this gives us a reason to dip into our stock of healing potions, at least.
You feel very good. You have 55 Grey Potions of *Healing*.
Good for 1000 HP apiece. We could've used a Potion of Life (good for 5000 HP), but ehh, save those for a rainy day. I don't trust the boss of this dungeon not to cheat.
But yeah, sometimes traps in ToME use the "." symbol instead of the "^" symbol. I don't know why, and it's a real dick move, because while the character may know they're there, the player doesn't. We'll automatically try to disarm any traps we move into, but our disarming skill is merely "Good" -- not remotely a guarantee against level-666 traps. And failing to disarm a trap can trigger it.
...Christ that thing did 1200 damage to us in one shot. 
Fortunately, we do get a reward from it: a staircase!
You enter a maze of down staircases.
...this isn't a reward. Oh, right, I'm in Hell.
Seriously, this is insane. We flee a pack of Aether Hounds, and wind up near a posse of "p" monsters, including a Patriarch who summons undead on us. So we flee that, and land next to a Dreadlord, his Dreadmaster minions, and a pack of Nether Hounds. They nearly kill us before we get our walls up, so we heal up and flee that and land up next to a Graveyard, full of awake and angry undead! Frankly it's amazing we're still alive what with all the blind Probability Travelling we've been doing; as it is we're down two *Healing* potions and have no clue where the stairs are.
Really, the only good thing about the Nether Realm is that there's no Spirits here.
Oh, thank all the gods. Tulkas, buddy, you're on my side, right? I've got a piety score of 123662, think I could trade some of that in for more stairs?
Thought not. Damn. Only 26 more levels of this to go! 
Fortunately this time the stairs are a bit easier to find. Of course, they land us next to a pack of Dreads:
I think I've figured out the problem with Black Hole. It's not so much that the monsters resist it that many more times. It's that they're starting from like +100 speed, so it takes a lot longer for them to get to the point where they're actually slow. Black Hole's slowing effect takes off 10 speed each time, I'm fairly sure, so that means it takes 6 effective hits just to reach speed parity, as we're at +41.
Fortunately, Dreads are fragile enough even at level 150 that Black Hole kills all but one of them in one casting anyway.
Hm, greater demon pit. Let's not go east. East is a bad direction. Let's go north instead.
You set off the unknown trap! You are completely startled by a sudden sound. You identified the trap as Trap of Drop Everything.
Oh, shit.
I hate ToME traps. Hate hate hate hate hate hate hate. Fortunately all of the stuff we lost is in the immediate vicinity; there's another trap that scatters your items across the dungeon level! Though that one I think can't touch your equipment. Also fortunately we already killed all the nearby monsters. This could have been a total disaster otherwise; we lost our spellbooks and our Cloak of Air (and ate 300 suffocation damage while recovering everything).
Phew, that's better.
Level 671 starts off predictably; we're too close to a posse of greater demons, who summon more demons on us:
In the turn we spend throwing up walls, we take 600 damage in melee. Level-150 monsters do not mess around.
Yes, this level looks friendly. 
Oh thank Tulkas. Sure there's some Nether Hounds in the area, but they can't pass through walls. And the Dread pack, which can walk through walls, is still asleep.
...big improvement. And we're only a fifth of the way through! I wish we were back in the Void. 
Staircase #7 is in a greater vault next to a pit of 95 greater demons. We activate one of our Genocide junkarts to take out the demon pit; it deals 1d4 damage to us per demon, but they're removed from the level. Unfortunately our junkarts take a long time to recharge -- 500 turns at normal speed, or about 2000 turns at our current speed level. Consequently we can't rely on them to make the Nether Realm a breeze.
Mages who worship Melkor can learn the Udun realm of magic, which includes the Genocide spell -- and you don't lose your ranks in Udun when Melkor turns against you because of destroying the One Ring either (which is necessary if you want to reach the postgame; if you wear the One, then Galadriel dies). So those mages can just trot around empty levels to their hearts' content. So too, I believe, can alchemists, who have a failure-proof way to recharge staves (and there are staves of Genocide, which we would have done well to stock up on before coming here, but oh well). But alchemists are cheeseballs extraordinaire, so who cares?
Level 673 starts out a bit hairy, but hark, what do my eyes see?
A staircase to the northeast! We can just throw up a Time Out (which thankfully does not remove stairs), and then Probability Travel straight to it.
And the luck keeps coming. I guess Tulkas just needed a little time to prepare. Thanks, buddy!
675 looks hairy, but fortunately there's stairs right around the corner of the western structure. And we're over a third of the way there!
On 676 we loot ourselves an interesting little dram:
Quaffing it gains us 4 more skillpoints, two-thirds of a level's worth. We get two more ranks in Spirituality for the saving throw, and 1 more rank in Stealth. And now we really are done with training; no more levels, no more Fumblefingers, no more mysterious extra sources of skillpoints.
...and we're staying the hell away from that pack of Quylthulgs in the northeast. If you've forgotten, Quylthulgs are stationary summoners, and with the way all the monsters here are level-150, it's easily possible for summons to act before we do. So we could get Ancient Dragons summoned on us, and have them all breathe and/or deal hundreds of melee damage before we get to respond.
676 is full of nuked regions, and annoyingly the stairs end up being in one of them. We can slightly expedite "exploration" of these areas by hitting them with Blast - Wall Destruction spells, but it's still slow going.
677 has the stairs in a fortunately sparsely-populated vault. I'm actually aiming us towards such vaults when we possible; the stairs seem far more likely to generate in a "room" (including vaults, destroyed areas, etc.) than out in the open, and it's harder for monsters to ambush us in vaults.
Floor 678...
Nope. Other direction. No graveyards plzkthx.
This, however, could be interesting:
This is a GCV, Great Checkerboard Vault, one of the most lucrative and safe vaults in the game. It's subdivided into a bunch of 1x1 cells, each of which has a monster and an item. Normally those are both massively out-of-depth (if you found one of these at level 20, it could have Great Wyrms in it, for example), but that's hardly relevant any more. I don't think it's possible for it to have any stairs in it, alas. Still, it should be an interesting experience, and it's remotely possible we'll find a gear upgrade.
You have finished the tunnel. The Greater demonic quylthulg magically summons greater demons. The Bile demon hits you. <2x> The Bile demon crushes you. You are covered in acid! Your Massive Iron Crown of Morgoth is unaffected! The Bile demon misses you. The Greater balrog hits you. You are enveloped in flames! The Greater balrog hits you. You are enveloped in flames! The Greater balrog crushes you. The Greater balrog touches you.
On second thought, maybe we shouldn't go picking fights while in literal hell. At least one of the demons dropped a Potion of Restore Mana, and another dropped a Sprig of Athelas. Might as well cure our plague, not that it's done anything to us since we got it several levels earlier. The Black Breath is harmless as long as your stats are sustained.
We simply turn on Probability Travel, take a step, and are outside the vault -- Probability Travel, like any normal teleportation, is not allowed to land you inside of vaults (although I think the Swap Positions power still works). And we land right next to some stairs! Lucky day.
Level 680...
I know I keep saying "Nope, fuck that", but seriously, this place is a landmine of awful encounters that could kill us in a single turn. Three Pit Fiends break out of that demon pit and chase us all the way across the level; we end up entombing the stairs with them only a few tiles away:
Fortunately they can't bore through rock. Unfortunately the next level isn't much of an improvement.
Yet another demon pit, and a vault full of demons and undead to the east. Note also the posse of jellies ("j" monsters) to our northwest. 
We end up using one of our Genocide junkarts on the demons; there's just too many of them. The Genocide backlash alone deals 300 damage to us, but that's easily healed up.
Levels 684 and 685 are both pretty kind, relatively speaking anyway. Level 686 gets us into a hairy encounter with a pack of Aether Hounds, whose breath weapons all deal max damage now that they're level 150 and have far more hitpoints. But we persevere -- mostly by entombing them all in rock. While we've needed to use some Potions of Healing and *Healing* in this place, we're actually gaining more potions than we lose, since they're semi-common items in vaults.
Then level 686 (two-thirds of the way there!) opens with this:
The Barbazu summons a demon! The Aether hound breathes force. You have been stunned. You are knocked back!
Within full sight of a positively gigantic posse of demons, all awake, as well as a large pack of Aether hounds. And because we've been stunned, our minimum failure rate on all spells, including Time Out, is 15%. I'd guess that spending another turn in LOS of all of these guys would mean death, even if we healed up (we're currently at 1048/1502 HP). This is a good time to invoke the nuclear option.
Really try 3 Scrolls titled "snefid bytrog" of *Destruction* {!*!*!*}? [y/n]
y
y
y
There is a searing blast of light!
You cannot fail to read scrolls. They're flat-out illegible if you're confused or blinded, but even when heavily stunned, your failure rate is 0% with them. Scrolls of Destruction are therefore perfect panic buttons. It's just a shame we only have 5 (well, 4 now) of the things. The Destruction effect litters our surroundings with walls and removes all monsters in the immediate vicinity. It's not as "dense" as Time Out -- monsters can still reach us -- but it buys us some much-needed time. We drink one of our 46 Healing potions (which cures the stunning), throw up a Time Out, and detect our environs...
Criminy. I don't like this level. We'll deal with it...next time! This update has gone on more than long enough. Next time: the bottom of the Nether Realm!