Flow of Knowledge

Flow of Knowledge is an end game caster trinket that was added with patch 3.0.8. Originally it appeared on the PTR servers as improving critical hit rating by 84, but was changed on release. It is primarily used for PVP due to its passive effects and resilience, however because of the high proc rate (10% chance) it can be combined with another similar trinket, such as the, for powerful effects.

To purchase a Flow of Knowledge (remember, you can only have one), you must earn s from battles within Wintergrasp.

Source
Flow of Knowledge is sold for 25 Wintergrasp Mark of Honor by

These vendors are only present inside Wintergrasp Fortress when the corresponding faction controls the fortress.