Exploring random zones via the exploratory far portal is a great way to gather more items or gold (dwarves love gold!), or to search for ingredients for the brotherhood of alchemist quests. However, if you explore more levels
after you killed the unique "boss", then this becomes a one way trip. Granted, most sane players shouldn't leave the portal back home, but, well, I don't know how sane ToME players are
.
I have two suggestions: (1) When you enter the zone that the unique boss is in, display a message like "The Rod of Recall begins to vibrate; there must be a powerful energy source nearby" to indicate that there may be a way home on this level. (2) If the player tries to leave the level, don't let them. Instead, display a warning message like "You dare not risk leaving your way back home"