Voldemort--speak not the name myth

[J]

No: HPFGUIDX 71676

There was a post on here a month or so back that mentioned a myth/lengend/
reference to a evil entity/essence that could be summoned up by a person to 
possess them and to transform the person from a human, mortal man into a 
creature with the makings to be immortal.  The secret to defeating/dispelling 
this essence fromt he possessed one was to know was once the name of the 
man--saying it and making the person recognize himself made him mortal 
again.  

Obviously, this sounds a little like Tom Riddle/ Voldemort.  I have been 
searching the net for a refernce to this myth or legend (I think it was norse/
germanic/or celtic in origin).  

Does anyone recall it or have any info to what I am referring??

Arya