You go to dark places so that you can get there, steal the trophy, and get out. That is much more important than to be psychologically safe. Frederick Bush
It's time for your hero to take the plunge, to cross the threshold into the underworld. It's scary, taking that first step over the doorsill or into the water or the cave, onto the ship or into the fire. But heroes have done this since time immemorial.They may not be happy about it, they may do it kicking and screaming, but they do it.
And by the time they get this far, they have a sense of what's involved in the journey. They've had the call, they've waffled about it, they maybe even have said no. They have someone (a mentor) who has told them what they have to do or told them how they did something similar. And even knowing the danger, they still do it, they still take that step and cross the threshold into the other world.
This moment tends to be an obvious one. Think of the Lord of the Rings where Frodo and Samwise run from home. Or Alice in Wonderland when she falls into the rabbit hole. Or think of more commercial fiction where stories often begin with a journey - a journey home, a journey to the past, a journey to a new life. Those thresholds aren't quite as obvious - but they're still there.
This is a crucial turning point in the story. Signal it, make it clear. Use some kind of symbol for a threshold: a door, an airport, a car trip, a rabbit hole, a new house or new life.
This piece of the story may not take long, but it's important. Make sure that first step is a good one.