How to survive after genociding the species

One thing that is very important in hell is fire resistance. The fire traps in Gehennom are particularly nasty because, unlike normal fire traps, they decrease your maximum HP unless you're resistant. You should also fireproof your armor (by reading a scroll of enchant armor while confused for example) and put all items that could burn or boil into a bag, so they won't be damaged or destroyed.

As to the level drainers: The best defense against level drain attacks is magic cancellation. If you have an item which offers level 3 magic cancellation, this will cause drain attacks to fail in most cases, making level drainers much more harmless.

For fighters it's also a good idea to have a silver saber as those are especially effective against demons and the undead (well, most undead - zombies don't care, but zombies aren't really dangerous at this stage anyway).

Also note that you can eat wraith corpses to gain levels.

General things you should do before you enter the end game:

• Get your naked AC down to 2 or lower with divine protection (you can get it down to 0 quite easily via donations).

• Have magic resistance and reflection.

• Be fast (or preferably very fast)

• Make sure all of your stats (except charisma) are close to or at their maximum

• Enchant most of your armor to +4 or +5.

• If you're a fighter, enchant your weapon to +6 or +7.

• If you're a caster make sure that all your important spells are recent in your memory or you have the spellbook in your bag of holding. You do not want to forget magic missile while battling The Wizard or Death.

• Make sure you have a means to detect portals (not important in Hell, but once you leave the dungeons, that's gonna come in handy and you do not want to have to go searching for gold detection scrolls after you have woken The Wizard).

• You should also have a reliable way to cure sickness (blessed unicorn horn or cure sickness spell) as there will be a couple of enemies who can make you terminally ill.

Cursed non-corpse food items, with the exception of the fortune cookie, are always rotten when you eat them. So the first course of action tends to be testing to see if it is cursed or not. See the top of Kaestur's answer to this question for details on that, but the simplest method is to drop it on the ground and see if your pet will walk on it.

Past that, it's primarily a game of luck. As with corpses, there is no visual indication of whether food is rotten. But unlike the deadly corpses, there is actually a check made when you eat the food. It is only made after it has been a certain number of turns since the generation of the item. So two rations of equal age can have different results when eaten.

Certain foods are more likely to rot than others. Eggs rot the fastest, to my knowledge. Fruit tends to rot earlier and more often than rations, although they are quite unfilling. The corpses of Lizards and Lichens never rot. To memory, lembas wafers don't rot over time, but if they do then they have a significantly lower chance of being rotten when eaten (and not cursed).