The spell steel wind strike (XGtE, p. 166) says that you:
vanish to strike like the wind.
(Emphasis added)
At first glance you might read this as an inconsequential part of the description. But strictly speaking there is no flavour text in spell descriptions. Vanishing implies being unseen, which (if we carry on the logic) grants advantage on attacks due to being unseen.
I've seen this argument appear in a couple of answers recently and thought it deserved a question of its own.
However, steel wind strike looks like a pretty strong spell without advantage, and granting advantage on all the attacks seems like something too important to leave to a bit of rules lawyering, which makes be doubt this interpretation.
Do you intrinsically gain advantage on all of steel wind strike's attacks?
Best Answer
RAW: you don't get any condition that grants advantage on attack rolls.
The description of the Steel Wind Strike spell says (XGtE, p. 166; emphasis mine):
The description says that you "vanish", not that you gain the invisible condition, nor that you are unseen in any term of game mechanics. The description of the spell would have specified such condition, as it happens for other spells or class abilities (some examples are down below). After having resolved all the attacks, you can teleport next to one target. Note that the description does not says You reappear and you can then teleport [...]: if the description assumed the meaning of "becoming unseen" for "vanishing", then there would be some indication of what happens after the spell ends. Moreover, in this way (vanishing=become unseen) there are no indications when some enemies have truesight or similar ability.
Hence, this spell allows you to attack simultaneously up to five enemies and then teleport next to one of your targets: during each attack you are not invisible or under some condition that grants you advantage on attack rolls.
Recall that spells only do what they say they do. For example, the Shadow Blade (XGtE, p. 164) spell says
Here it is explicitly said that you roll with advantage (under certain environment conditions). Another example is Zephyr Strike (XGtE):
Spells that grant one or more conditions which may give advantage on rolls explicitly stress out such conditions (as per Invisibility spell, Basic Rules , pg. 254). Other spell, on the other hand, have a more general description, but searching in the PHB you can find the correct interpretation. For example, Darkness produces...well, darkness, and you can find the consequences of this spell here:
Furthermore, there are some uses in the PHB of the verb vanish and when its equivalence in game terms is required the text is pretty clear: