The true strike cantrip can be cast as an action to grant yourself advantage on the first attack roll against the target on your next turn.
The Samurai fighter's Fighting Spirit feature description says (XGtE, p. 31):
As a bonus action on your turn, you can give yourself advantage on weapon attack rolls until the end of the current turn.
The Rapid Strike feature description says (emphasis mine):
If you take the Attack action on your turn and have advantage on an attack roll against one of the targets, you can forgo the advantage for that roll to make an additional weapon attack against that target, as part of the same action. You can do so no more than once per turn.
Say I have a level 20 Samurai fighter, and I've taken the Magic Initiate feat to learn the true strike cantrip. Can I cast true strike on turn 1, then on turn 2, use the Rapid Strike feature to turn it into 2 attacks, giving me 5 regular attacks + 4 more from Action Surge, then use my bonus action to use Fighting Spirit?
Are all 9 attacks all at advantage? The description of Rapid Strike says "if you take the Attack action on your turn". As it doesn't specify when on your turn you're doing it, is it allowed to then give the attack advantage again?
Best Answer
No, you either get an additional attack or advantage on the first roll
If you use Fighting Spirit before taking your first Attack action, your first attack would have advantage applied from two sources. But advantage and disadvantage don't stack. That is, you can't have "double advantage."
So by using Fighting Spirit, your first attack (along with all your other attacks) gains advantage, which it already had from True Strike. Once you decide to take a second attack instead of advantage, the advantage is foregone as a form of payment to activate that ability.
Best case, all attacks except the first one would have advantage (the second attack created by Rapid Strike would benefit from Fighting Spirit).
As for timing, you can't give advantage to attacks that have already been made. So you can't use Fighting Spirit after the attacks you want it to apply to. You have to use it first. But this is not a problem because you can simply take your bonus action before your other actions. This would make True Strike unnecessary (even wasteful).