I don't know what the exact condition for unlocking it is, but I think its during the 10,000 silver gathering phase, once all the guild missions have been done, successfully or failed, with only the repeatable hooligan mission remaining, Serena decides she needs to get stronger to face Ganelon as she doesn't think just the 10,000 silver will end the matter (possibly means it's restricted to the route where she lost against Ganelon initially?) and asks the guild master for permission to go into the various monster areas again. Losing against the orcs a 3rd time leads to an unmarked bad end choice selection if a bloody cleaver is interacted with.
I haven't explored all the lose sequences yet, but the goblins have an extended confinement scenario instead of the guaranteed rescue in the initial goblin mission, the worf confinement is unchanged, and the Stallion scene is repeated where you can control speed and progression.