Ruthless Deathfang

• WWizards of the Coast2015-02-25
  Official Ruling
  Notably, the “legend rule” does not cause any permanents to be sacrificed. They are simply put into their owners' graveyards.
• WWizards of the Coast2015-02-25
  Official Ruling
  Ruthless Deathfang's triggered ability will trigger whenever you sacrifice any creature, including Ruthless Deathfang itself. It will trigger whenever you sacrifice a creature for any reason, including to pay a cost or as the result of a spell or ability (such as an opposing Ruthless Deathfang's ability).

Rulings via Scryfall