If a Champion is destroyed and removed from the game, and then a card would return a destroyed Unit from the discard pile, does the Champion return or remain out of the game?

A destroyed Champion remains out of the game and does not return if a card would return a destroyed Unit from the discard pile.

According to page 16, "Destroyed Acolytes are moved to the Discard Pile, while destroyed Champions are removed from the game. To represent the destruction of a Champion, put their card face-down in their controller’s Champion Area."
Page 12 clarifies that a Champion card is never shuffled into a player’s deck, and if destroyed, it is put back into the Champion Area, face-down.
The rulebook distinguishes between Acolytes (which go to the Discard Pile) and Champions (which are removed from the game) when destroyed.
Therefore, if a card effect would return a destroyed Unit from the Discard Pile, it would only affect Acolytes, since destroyed Champions are not in the Discard Pile but are instead removed from the game.

Source: pages 12 and 16.

Source: Core Rulebook (pp. 12, 14, 15, 16, 20)

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