Waterdeep:Knot in the Weave

Before Waterdeep, and before Halaster, the ancient elven city of Aelinthaldaar stood, capital of the elven kingdom of Illefarn. When the elves left, they used powerful magics to erase all traces of their capital city. As a side effect of their casting, they created a knot in the Weave underneath the mountain.

This snarl sometimes causes madness to one degree or another in the minds of mortals, which usually manifests as an obsession with the mountain and what lies under it. Not all who dwell in Undermountain succumb to the madness caused by this tangle of magic, but any who spend a considerable time below the surface risk falling victim to it.

Mechanics

Each day that you spend in Undermountain, you must make a Wisdom saving throw. The DC of the save is equal to the deepest level of Undermountain you have reached since the last time you visited the surface. If you are an arcane spellcaster, the DC increases by 2. Arcane spellcasters are anyone with at least 1 level of artificer, bard, sorcerer, warlock, or wizard; at least 3 levels of fighter with the Eldritch Knight subclass or at least 3 levels or rogue with the Arcane Trickster subclass; have a racial feature which grants you a spell from an arcane spellcaster's spell list (such as high elf gaining a wizard cantrip); or have a feat which grants you a spell from an arcane spellcaster's spell list (such as aberrant dragonmark gaining a sorcerer cantrip and 1st level spell).

On a failed save, you gain 1 level of Weave Addiction.

The ancient Netherese that settled beneath Undermountain could hear the call of the Knot. Knowing what sort of fate was in store, they placed wards upon their territory. To this day, this abjuration magic permeates Skullport. Creatures residing there automatically succeed on their saving throw to increase their Weave Addiction. A creature already afflicted with Weave Addiction, however, cannot make a saving throw to reduce their Weave Addiction.

Reducing Weave Addiction

If a creature with Weave Addiction returns to the surface and takes a long rest, they may repeat the saving throw (DC equal to the deepest floor reached since the last time they took a long rest on the surface). On success, they reduce their Weave Addiction by one level. Notice that with 23 floors to Undermountain, this save may eventually become impossible to meet for the more weak-willed.

A creature can be subjected to a spell of greater restoration to remove one level of Weave Addiction, but the caster must contend with the forces of the Knot itself. The caster must succeed on an ability check using their spellcasting ability (DC 3 + the current dungeon level + the level of weave addiction). Failing the check consumes both the spell slot and material components used and causes the spell to fail.

Weave Addiction Levels

Below are the effects of Weave Addiction that you are aware of.

Level 1

Allure. You are charmed by Undermountain itself, as if by a charm person spell. Checks related to convincing you to return are made with advantage.

Level 2

Disheveled. You have disadvantage on all ability checks made while outside Undermountain.

Level 3

Dissonance. You have disadvantage on all saving throws while outside Undermountain.

Level 4

Withdrawl. You cannot gain the benefits of a long rest unless you are within Undermountain, or you are on the surface and within 1 mile of the Yawning Portal.