Compiled from my replies in a different thread for completeness.
I've written a custom move for when/if the PCs dabbles in the drug Myth, which is a Threat (Affliction: Delusion, impulse: to dominate people's choices and actions).
When a character takes a hit of Myth, roll +cool. The MC tells them something they get an impulse to do.
On a 10+, they take +1forward for as long as they're actively working towards their impulse and mark experience when they've fulfilled their impulse.
On a 7-9, they mark experience when they've fulfilled their impulse.
On a 6-, if they're not working towards their impulse, they have to Act Under Fire - on a 10+ they do it, on a 7-9 they're given a bargain, on a 6- they fail and the MC makes as hard of a move as she likes.
For each hit of Myth a player character takes, increase the Myth Addiction Countdown Clock by one section.
Myth Addiction Countdown Clock
Purpose: to track how the Myth Addiction is spreading in the holding and determine what happens at specific points of the holding's descent into madness.
Custom MC move: advance the clock by one section by introducing an NPC with these effects into the narrative, or by showing an existing NPC subjected to an effect higher up on the clock. Descriptive on each level, prescriptive only insofar that narration of a higher state increases the section by one step (i.e. it doesn't skip directly to that step).
Custom player move: decrease the clock by one section by combatting the drug and use of the drug through radical action (e.g. cut off the supply lines of the drug, kill a prominent proponent of the drug, eradicate a cult that's formed around the drug, implement and enforce policies about the use of the drug).
Section 12-3 - Myth users are creative and productive in a manner bordering on manic, but suffering from some tunnel-vision and eccentricity in their thinking which impact their functioning in society somewhat. Users do not suffer as strong effects of heavily rationed resources, and are able to go for days on little food, water, and sleep.
Section 3-6 - Myth users' personalities alter in subtle but significant ways, and their individual undertakings become less and less in line with the needs of the holding as a whole and displaying even greater eccentricity (e.g. Rum decorates the interior of her home with the skulls of the deceased, excavated from the holding's graveyard, dangling from leather cords from the ceiling; Jackabacka insists that dirt is a nutritious additive to meals and starts using it as a main ingredient in all his dishes).
Section 6-9 - Some myth users have major breakthroughs recognized by the population at large (e.g. Rum claims to be able to speak with the dead in her home, and as far as anyone can tell she does receive accurate information; Jackabacka's dishes start to have nourishing effects on its imbibers, even though they taste horrible), however, heavy users exhibit increasingly strange and antisocial behaviour (e.g. Rum declares that the dead have told her that Mother Superior must be made to suffer; Jackabacka forcing dirt down the gullets of non-believers).
Section 9-10 - Due to either withdrawal symptoms or an overdose of Myth, a named NPC dies or causes someone else to die horribly (e.g. Jackie shoots an innocent and laughs hysterically at the pattern their blood forms as it pools on the ground). Myth users become beholden to their supplier and will do almost anything for another hit.
Section 10-11 - Erratic and destructive behaviour is introducing chaos to the holding. Non-users are starting to behave as erratically as users due to constant breach of routine, paranoia, and having to protect themselves from physically violent users. A small-scale riot, easily controlled by a well-organized gang of non-users, erupt across the holding, instigated by individual users. Its participating members do not form a mob as there is no cohesion.
Section 11-12 - Users of similar sources of Myth band together in cult-like units to overthrow the holding under similar but erratic, non-sensical principles (e.g. the Followers of Davok's Word wants to sacrifice Mother Superior to the scorpion spirits, the Lengmar Disciples starts construction on a roof over the holding to banish the sun, the Mothers of Meredith wants to slay all males in the name of their daughter patron). Each one of these cults act as a gang.
My own game's countdown clock is currently in the second section. I'm interested in any feedback you may have on this.