In this chapter, it is only the character who turned the chapter card who cannot use MIGHT. Think of it as all the MIGHT symbols disappearing from their character dice until the end of the chapter. If they roll a MIGHT on their character dice it is considered to be a blank. However, if the affected player rolls a DOUBLE MIGHT, the shield symbol shown on the dice does still apply - they have rolled a blank BLOCK.
All other players can fight as normal, so it falls to those other characters to summon their collective MIGHT in order to survive the combat.
The relic known as Rengorn’s Rune (DCSE-IC11) allows a player to modify their character dice to any result, but since the MIGHT symbols are considered to have vanished from their character dice, the relic is of no use to the player in this chapter.
However, the Brew of Might potion (DCSE-IC05) can be used by the affected player to generate MIGHT in this chapter, since it does not involve the player’s character dice in any way.