Just before getting married, the two went back to the town where they grew up, to search for a certain thing.
That thing was "the cave", which they knew the town elders had been secretly discussing.
Rine, who had the skills to rival even the best of the knights at the castle, set out with Zakk to look for it...
...and at the hands of the demons at the end of the cave, they were driven mad.

[From DLsite English]