The Deadra heart is one of the ingredients needed for the creation of any deadric weapons/armor. it is a very rare ingredient, and very hard to obtain, but you can find them if you know where to look.
One place where you can find deadra hearts for certain is justoutside and inside the Shrine of Mehrunes Dagonthis place is part of the quest for Mehrunes' Razor. Be carefull, thay attack in pairs. the deadra at this place do respawn if you come back. you can get four at a time if you kill the deadra outside and inside.
Daedra hearts are also available from Enthir who can be found on the second floor of the Hall of Attainment at the College of Winterhold. He has two daedra hearts at a time and these will respawn every couple of days.
