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 a bunch of scenarios I can think of, that hinge on whether the attacker is moving between strike to be next to the creatures or not, hopefully by listing them all I can shake out an understanding of what is happening.
Also for reference, I'm aware of the question asking Do you get advantage on the Steel Wind Strike spell's attacks because you "vanish"? as well as Do Steel Wind Strike's attacks have advantage or disadvantage against distant prone targets? but I think some of these examples might require a different answer.
What if any are the side effects of these situations?
- Targeting a creature inside the area of an Alarm Spell (Basic Rules , pg. 211), without starting or ending up inside the area - set the alarm off or not?
- Targeting a creature 5 foot inside the range of a caster who knows Temporal Shunt (so that all squares around the target are in range but you are not.), even though you start and end outside of range - the Steel Wind Striker can be targeted or not?
- You have Crusader's Mantle (Player's Handbook, pg. 230) and the aura would overlap with a non-hostile who readied to make an attack as a reaction, but only 'during' the attack not before or after - the readied attack gets the damage bonus or not?
- Targeting a creature with Fire Shield (Basic Rules, pg. 242) or Shadow of Moil (Xanathar's Guide to Everything, pg. 164) if it starts and ends 5ft or 10ft away- those spells can lash out with fire or shadow or not?
- Targeting a creature within 5ft of another creature with the Sentinel feat. but not starting or ending in range of the Sentinel - the sentinel can make an attack as a reaction or not?
For the sake of clarity, I'm not talking about the situation where the caster of Steel Wind Strike starts or ends that spell in range of, or inside the area of the above effects/spells, but where their targets are inside some area or range.
These feel like they cover a lot of the fundamentals for what can react to the attack from a Steel Wind Strike. It also differentiates between the vanishment being visual (similar to invisibility) or physical (perhaps similar to being ethereal/spectral but less well defined).
Does this tell us anything about the position of the caster of Steel Wind Strike when they make each attack?
*: Or your metric's** equivalent
**: No, not the metric system, the Distance Function