In Episode 3 of Rise of Tiamat, the players enter the Tomb of Diderius. Diderius has become a
and as such his lair (the Tomb) should show this legendary monster’s Regional Effects (enumerations and emphasis mine):
A mummy lord’s temple or tomb is warped in any of the following ways by the creature’s dark presence: (1) Food instantly molders and water instantly evaporates when brought into the lair. Other nonmagical drinks are spoiled—wine turning to vinegar, for instance. (2) Divination spells cast within the lair by creatures other than the mummy lord have a 25 percent chance to provide misleading results, as determined by the DM. If a divination spell already has a chance to fail or become unreliable when cast multiple times, that chance increases by 25 percent. (3) A creature that takes treasure from the lair is cursed until the treasure is returned. The cursed target has disadvantage on all saving throws. The curse lasts until removed by a remove curse spell or other magic.
However, Diderius is special in that:
When Diderius died, those who honored him in life transformed him into a special mummy lord whose magic pervades his tomb. Since Diderius is neutral rather than evil, the area lacks dark magic common to other mummy-lord tombs.
This might lead us to believe that none of the standard Regional Effects for a legendary monster of his type are actually in play while in the Tomb. Nevertheless, as areas 5 and 12 explicitly state
The well holds water still, but the regional effects Diderius imposes on his lair causes it to evaporate within a few rounds of being hauled up into this area.
so at least one of the Regional Effects in fact still operates.
How do we know whether the other Regional Effects are
How do we know whether the other Regional Effects apply in the Tomb?