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
Best Answer
Are you ending outside of the forcecage?
First, it's important to note that steel wind strike doesn't say that the caster teleports next to the targets; it just says that they make melee spell attacks. Obviously, it being a melee attack strongly implies that the caster is somehow close to each target, but it doesn't actually say that. The spell just says that you choose the targets, and make attacks against them.
At the end of the spell, the caster explicitly teleports somewhere. We'll come back to that.
The cage mode of forcecage doesn't block spells, so as long as the caster and all targets are in the appropriate range, the presence of the forcecage around any of them shouldn't affect the spell.
What forcecage does block is leaving the cage by teleportation. So if the caster of steel wind strike is (a) inside the forcecage, and (b) intends to use the explicit teleportation part of the spell to end up outside of the the forcecage, then this clause kicks in:
My reading of this is that when a caster inside the forcecage wants to steel wind strike their way out, they make the Charisma save. If they fail, then the spell fails totally -- no five attacks -- and they stay in the cage.
If the caster wants to steel wind strike a bunch of targets outside the forecage but not teleport out at the end, then as written, nothing in the either spell description prevents this.
Of course ...
It's clear from an unofficial Crawford tweet that the intended flavor of steel wind strike is that the caster teleports from target to target. So a DM could certainly rule that any use of the spell on targets outside of the forcecage, or including a target inside the cage but not having the caster remain there at the end, involves teleportation that the cage might interfere with.