No Errata
Q: How do I use Tireless Assault with an Attack action that targets more than one enemy, such as Severin Steelheart’s Mighty Swing?
A: An attack that targets more than one enemy is treated as a number of individual Attack actions performed one after the other, and each of those Attack actions is judged as succeeding or failing based on whether or not its target is damaged. If one of the Attack actions made as part of a Mighty Swing fails, you can play Tireless Assault (this can interrupt the Mighty Swing, and indeed will, unless the Attack action you are reacting to is the final Attack action made as part of Mighty Swing) to make another Attack action that targets the same fighter. This could be another Mighty Swing (allowing you to target other fighters as well). Once you have resolved the reaction, if it interrupted Mighty Swing, you would then finish resolving Mighty Swing (unless that was no longer possible).

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