“When The Guild attacks a region that has a Crysen, it deals damage equal to # of Crysen in play.”
When attacking a region with Crysen with the Battle action, does the damage replace the 1 damage from the Battle action, or add to it? (replace)
The damage replaces the standard 1 damage that the Battle action would normally deal. It does not add to it. So if there are two Crysen in play and the Guild uses the Battle action to attack a region with a Crysen in it, the Guild deals 2 damage.
Does having multiple Crysen in a region multiply the damage dealt by The Guild? (no)
The Crysen ability works the same whether the region being attacked has a single Crysen or multiple Crysen. Having multiple Crysen in the region does not multiply the damage dealt. If at least one Crysen is in the region being attacked, the amount of damage the Guild deal is equal to the total number of Crysen in play.