How about the opposite of my last post's idea of a spell.
With a kiss a mage "hides" a bit of lifeforce with the kissed (1 hitpoint). The mage may kiss as many individuals as s/he has lifeforce (hitpoints). As long as the kissed remain alive the mage will be unkillable. If wounded, the kissed will go into a coma at the last bit of lifeforce (1 hitpoint left). The mage will know whether any of the kissed are living, dead, or in a coma, but not necessarily where they are.
What level would you make this?