The Mobile feat (PHB, p. 168) says:
When you use the Dash action, difficult terrain doesn't cost you extra movement on that turn
The monk's Step of the Wind feature says:
You can spend 1 ki point to take the Disengage or Dash action as a bonus action on your turn…
Due to the wording here, would that mean that if a Monk with the Mobile feat uses Step of the Wind to Dash, are they not affected by difficult terrain?
Best Answer
Yes
When you spend a ki point on Step of the Wind (and choose to Dash) you're taking the actual Dash action (as a bonus action). Since Mobile's effect applies to when you take the Dash action, Step of the Wind's Dash would mean difficult terrain doesn't cost you extra movement for that turn.