Fernswarthy's Basement, located in The Ruins of Fernswarthy's Tower, is just that. A basement. Except that it's a basement without any apparent bottom. The obstacles you must overcome to continue downwards become progressively harder as you go. Some rooms contain a monster to fight, and some contain tests of your Muscle, Mysticality, Moxie or endurance. Every fifth room, however, gives you the chance to boost one of your stats. Additionally, every hundredth room, you will receive rare and powerful consumable items (or other useful items, if you descend far enough). Fernswarthy's Basement becomes available once you find the trap-door during The Wizard of Ego quest.
In most cases, the stat requirements to pass a floor are directly stated. For example, if you are required to dispel 468 mops, you will succeed with 468 mysticality and higher. Some monsters will display their strength as well. "The Beast with 468 eyes" will attack and defend as a level 468 monster. Some obstacles do not show their requirements, but simply increment each time you encounter them. These adventures will still scale according to the floor normally.
The stat requirements on any given floor are roughly equal to (floor)1.4, and the levels of the monsters can scale up to 2*(floor)1.4. The following table lists the approximate buffed stat required to reach each stat-boost floor. According to the Dressing For Success model, any player can achieve at least a 500% buff for each stat, the third column gives an approximate minimum unbuffed stat. A player with a main stat of 126 can use commonly-available equalizing potions, clothes, and stat-boosting potions to reach the 100th floor of the basement; advanced players and those with nice toys can obviously get by at a comparatively lower main stat.
See Maximizing Your Muscle for a guide to obtain the highest muscle, or refer to Muscle Modifiers for a comprehensive list of all muscle modifiers in the game.
See Maximizing Your Moxie for a guide to obtain the highest moxie, or refer to Moxie Modifiers for a comprehensive list of all moxie modifiers in the game.
See Elemental Resistance for a comprehensive list of all elemental resistance increasers in the game. Being in the appropriate elemental form will reduce one of the damage types to 1, but make sure that you do not use the form that doubles the other elemental damage type. For example, using sleazeform against Singled Out will reduce sleaze damage to 1 but double cold damage.
One of the stat gain rewards will appear on every floor that is a multiple of five. The only exceptions are the one-time reward rooms which occur on the first five multiples of 100.
Stat tests at floor N require about N1.4 buffed stat.
To get to floor 400 at level 30 (845 mainstat), all that's really needed is a 420% boost to all stats, easily attainable from reagent potions alone, and an equalizer potion.
To get to Floor 500 at level 30 requires about a 610% boost to all stats and an equalizer reagent potion.
Clever use of Elemental Phials and Maximal Resistance Items such as the Donkey flipbook will greatly help non-myst classes with the elemental tests. However, mysticality classes will generally have a much easier time with these tests (especially with skills that provide elemental resistance), and also with the MP drain test.
Stat tests are not necessarily the limiting factor for everyone. Players without Torso Awaregness or Tao of the Terrapin may find themselves stopped at the Gauntlet; without Wisdom of the Elder Tortoises, at the Festering Powder test; without an uncapped Myst damage spell, at high-HP monsters.
A not-so-mini guide for diving in the basement is available.
When beaten up you get the message "You're too beaten up to accomplish anything in the basement. Unless bleeding counts as an accomplishment."
When falling down drunk, the following message appears: "I know you've got fond memories of stumbling around your parents' basement in a drunken stupor, but this isn't that kind of basement.
When out of adventures you get the message "You don't have time to screw around in Fernswarthy's Basement."
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