Found a hidden mechanic for Warriors, and I'm sure there's one for other classes as well.
https://i.gyazo.com/dcfada876836bd79...929a3c5588.gif
(note the left most buff, gif cut off, but death occurs on expiration)
https://i.gyazo.com/b9b51eb112219050...9ea9b05c3b.png
https://i.gyazo.com/517a5af7d18dbb6c...72bbf002ed.png
(huge heal, attributed to Odyn's Fury as the effect which triggered the KB)
http://legion.wowhead.com/spell=2188...-of-the-valkyr
http://beta.wowdb.com/spells/218826-trial-by-combat
http://beta.wowdb.com/spells/218822-gaze-of-the-valkyr
(15m CD, not displayed in game)
Basic mechanics for those uninterested in clicking links:
Buff procs on attack that would kill the player, giving them 8s of immunity. If they kill the target that triggered it within that 8s, they receive a substantial heal, if not they die. Not sure if it requires the killing blow (thereby limiting it's usefulness in raids/group content) or just for the target to die, but probably the former.
*The buff also automatically triggers Enrage, which refreshes itself halfway through (giving 8 full seconds of Enrage), and generates 10 rage every 2s.
Other possible class effects:
(Unconfirmed! I found these by digging through spell data, so it's possible they could be associated with other effects such as trinkets, encounter mechanics, and so on. Only the Warrior effect has been confirmed so far.)
Hunter: http://legion.wowhead.com/spell=2193...eagles#summons *Verified.
Warlock: http://beta.wowdb.com/search?search=...gorized-spells
Druid: http://beta.wowdb.com/spells/218805-elunes-redemption
Paladin: http://legion.wowhead.com/spell=1853...t-of-vengeance
- - - Updated - - -
That was quick; apparently this was supposed to be a secret. Whoops.
http://us.battle.net/wow/en/forum/to...16?page=11#202