It's detailed the same place "judge the living" and "freeze them with a touch" are detailed.
That place is inside your head. And, later, in the narrative.
Every monster has moves that describe its behavior and abilities. Just like the normal GM moves, they're things that you do when there's a lull in the action or when the players give you a golden opportunity. As with other GM moves they can be hard or soft depending on the circumstances and the move: a move that's irreversible and immediate is hard, a move that's impending or easy to counter is soft.
-- "Elements of a Monster", from the repo
None of the moves you make as a GM function anything mechanically like the player moves, a term which includes the basic moves, playbook moves, and custom player-facing moves you develop as part of adventure prep. As a GM you're never rolling dice and consulting a results chart, or picking options from a limited menu of choices. When you "make a move" as a GM, you're not even supposed to be calling attention to it as a move; it's just a part of your narration designed to give the players something to react to.
When you're developing monster moves (or using existing monsters) as part of your prep, what you're writing down are prompts for yourself when you have to narrate something for the players to respond to. Consequently, you should look at every monster, danger, or location move you're prepping and take a moment to think about the kinds of basic GM moves it will let you make.
For example, since "show the true face of death" is the move the spirit of judgement gets because it's terrifying, it's got some pretty obvious hooks to separate them (you flee blindly into the darkness), put someone in a spot (you flee blindly toward the cliff's edge), and deal damage (the true face of death is seared into your senses, mark confused).
Prepping Custom Moves
It seems a little strange to me to talk about "custom" monster, danger, or location moves, though the book does use that language when talking about writing your own variant prompts as your prep demands. Again, just keep in mind, whenever you're customizing something from the book, that what you're writing is a prompt for yourself to make a variety of GM moves.
However, prepping a monster move might also suggest that you should be writing a player-facing custom move to deal with the consequences of confronting it, if for instance you want to present a more specific way to act in the face of fear than just defying danger (with apologies to Ghost Lines and Stonetop):
When you first glimpse the true face of Death from a distance, choose your worst instinct:
- freeze up, leave yourself open
- panic, disengage, flee
- collapse, let go, give up
- rage, lose control, lash out
Then roll +WIS. On a 7+, you hold together. On a 10+, you can also help someone else hold together. On a 6-, if someone can help you, you hold together; don't mark XP. Otherwise, the GM tells you where your worst instinct takes you.
When you first confront the true face of Death up close, choose your worst instinct as before and roll +WIS. On a 10+, you hold together. On a 7-9, the GM will tell you where one of your other instincts takes you. On a 6-, the GM tells you where your worst instinct takes you.
Mortal minds are not meant to remember the true face of Death. When you rest for a few hours, Make Camp or otherwise, treat your next encounter with the true face of Death as your first.