As player moves

Write a custom player-facing move to reflect the hazard (or part of it), like "When you breathe in the horrible stench of the Maw, roll +CON…." When the PCs encounter the hazard and the move becomes relevant, share it with the players. Resolve it like you would any other player-facing move.

A player-facing move is a great way to establish and communicate stakes. You can tell a player that they'll have to Defy Danger to leap across the chasm, but unless you also tell them what will happen on a 10+, a 7-9, and 6-, they don't really know the risks. With a custom move, you can bake the stakes right in.

Player moves are also a good way to handle "weird" hazards, like magic that messes with their minds or that follows strange rules of cause-and-effect. A player move can abstract these situations and give them a vague, dreamlike quality.

Remember: everything begins and ends with the fiction. If the PCs come up with a clever way to bypass a hazard or its trigger, then your move might no longer apply. That's great. Don't negate their choices, their creativity, or their abilities just because you've got a cool custom move you want to use.

See Writing moves for guidance.