"Play at the beginning of the challenges phase."

We should read this literally or is it mistake?

Rogue30 said:

Are you really asking, or just pointing out a mistake?

Because it is a triggered effect "Challenge" action, the text comes down to "play at the first available opportunity." If you want to get really technical and create some rationalization, all "at the beginning of the phase" effects do happen at the first available opportunity rather than "at the beginning." Passive effects happen in the same (framework) action window as the beginning of the phase - unless Shadow windows are involved - so it looks a lot different, of course, but the phase has technically already begun when they initiate. And yes, of course, that is pure rationalization - as I already mentioned.

It should have said "play before any challenges are initiated in the Challenge phase," but since it is clear exactly when it should be played, I wouldn't hold my breath waiting for errata.