Joan of Arc comes from the Hundred Years War campaign.
Joan is a Flag Bearer unit. Flag Bearers increase your gambit heroes morale by 34 and reduce enemy gambit hero morale by 5 each turn. This ability is listed as a spell, however it does not have a fail rate nor is it's effect strengthened or reduced by any modifier in the game.
Joan of Arc is the weakest of all Flag Bearers and should be replaced with any of the others as long as you don't need an additional Flag Bearer with Joan being the only option.