When you use the servants of kallandra optional rule, dead players will control monsters on the board and may put more monsters on the board (if there are less monster than players alive on the board).
Monsters ignore doors, portcuils and tile's special effects, but me and my friends verified in our last games that is difficult for a monster to reach one character if the monster isn't able to pass through walls OR move on unexplored places. Rules don't allows monsters to do that, what do you think about it?

The only doubt is that the dungeon is so hard to escape from even without monsters passing through walls (like zombies in last night on earth) and with this homemade rule it could became harder, nearly impossible.

Simply put, it's a terrible idea for multiple reasons beyond even those you mention. The fatality rate in DQ has always been high, and it should be.  This notion is just to pacify those who got knocked out and let them try to escalate the chance of a no-win ending.  That ending is always possible, even likely, in DQ.  Putting the escalation into the hands of those who've already lost is not an option to be applied to DQ.... it's pandering.

JCHendee said:

ahahah, you're right JC! surely it wasn't a great idea...