When I go inside the forge, after the first battle, my sovereign ends up alone against a tonne of monsters.

For this battle his maximum hp changes to ~500, but his initial hp stays, so I end up with something like 45/~500. And of course it's impossible to beat them like this. Maybe get healing spell, cast it on yourself for a few rounds, while they're getting closer, but even then, it's still seems almost impossible.

It looks like something else should happen to Relias when he goes inside, making him much stronger, maybe he should have gotten healed, gotten extra spells, more damage, e.t.c..

Am I missing something?