Arkham Horror, Amun Re, R-ECO, Ora Et Labora, Napoleonic Wars
The rules are clear. Bold emphasis is mine.
Rules, Section 17.5 Champions and Sucessors wrote:
A player who wishes to keep his Champion status may only attack Successors and
Usurpers. They lose the Champion status the instant they attack another
player who is also a Champion (and not the Usurper). Indicate
the loss of this status by flipping over the marker to its Successor (0
L) side. Your Legitimacy rating is adjusted before the battle starts,
so this can effect Royal Army Macedonians in the battle.