The spell Steel Wind Strike (XGE, p. 166) says:
You flourish the weapon used in the casting and then vanish to strike like the wind. Choose up to five creatures you can see within range. Make a melee spell attack against each target. On a hit, a target takes 6d10 force damage.
The range is 30ft, so that would make any creature in a 30 ft circle* around you a valid target, in theory.
But are these valid targets?
- a creature inside a Forcecage (Basic Rules , pg. 243) with the prison set to 'cage', while you're on the outside
- a creature outside a Forcecage with the prison set to 'cage', while you're on the inside
Forcecage has some rulings in it about teleportation and entering it, even via the ethereal plane, so I'm not confident they are valid targets? The 'box' setting would definitely block spells, as it's
[...] a solid barrier that prevents any matter from passing through it and blocking any spells cast into or out from the area.
But even so, there's no physical way inside the prison when it has the 'cage' setting either.
*: Or your metric's** equivalent
**: No, not the metric system, the Distance Function