This quest is offered by [npc=111243], and grants the [currency=1273] - the new bonus roll currency at Legion launch. These seals require various currencies to obtain.
[npc=111243] is located at (56, 67) in the area outside of the Violet Hold (to the right side near the pools as you are facing the Violet Hold entrance).
[currency=1273] will be usable in all Mythic dungeons, in the new Tier 19 raids [zone=8025] and [zone=8026], and for the new world bosses (Ana-Mouz, Calamir, Drugon the Frostblood, Flotsam, Humongris, Levantus, Na'zak the Fiend, Nithogg, Shar'thos, The Soultakers, and [npc=112350]).
As with recent currencies, only 3 per week will be obtainable (with potentially rare exceptions), and the hard cap will be 6 seals.
The currency can be obtained from quests that require a certain amount of gold or [currency=1220] (similar to the Warlords of Draenor model). These quests are available from [npc=111243] outside of the Violet Hold at [58,67] in Dalaran (Legion version). Your second and third seals purchased with the SAME currency increases progressively in price - this is noted in the chart below.
Some of this information may not be final, and is subject to change
As in Warlords of Draenor, you can mix and match which currency you use for seals (some with gold, others with resources) to a maximum of 3 seals total.
If you only buy 1 or 2 seals with a specific currency, the quests will reset the following week, allowing you to spend the lowest amount again (you only pay more for the second and third seals you buy within the week).
NOTE: In Legion, it will be possible to trade Personal loot items if you already have a better item in that slot. It remains to be seen whether this will apply to bonus rolled loot.