Just an alert to those on Touch of a Titan, as the lion's share of people will be hitting 50 for the first time tonight.
When you complete the quest for the first time - ALL excess essences over 50 will be discarded as they are stored as a hidden quest currency, not a real one.
So - if you are close to 50 now, do NOT open up your M+ cache first thing tonight, get yourself just over the 50 first (via a PvP weekly, or a few LFR NH kills, or a WQ) depending on how far away you are.
Most people will lose a couple of essences, that's unavoidable - but it would really suck to lose 15 from the M+ cache.