Burial mask drop at critical level
If the player is at their lowest amount of sanity after the haunt starts and the item is dropped or stolen they immediately lose the +1 sanity benefit the item gave them and would go insane (die).

I couldn't find any references to this in either Legacy or Betrayal rulebooks but it seems that all heirloom effects belong to that heirloomed item and to benefit from those effects you must have it in your families possession.



I actually reached out to Avalon Hill asking this question and fantastic support rep Frank sent this response:


  If an item grants a benefit and it doesn't specify permanent benefits,
  "discard" or "destroy" this card upon use, then the benefit is lost if
  the item is lost. Benefits granted by items only last as long as you
  have the item, unless otherwise stated. Because of the above concept
  it does seem the character would die if that item was keeping them
  alive i.e. only 1 sanity remained and they lost the item. However,
  keep in mind if the rules are not specified in the rulebook you and
  your group can decide what makes the game most fun.