However, if you complete the quest but do not turn it in...endless pet experience. For now. This might be something Blizzard addresses and/or nerfs.
A WarcraftPets user discovered a really neat item on the Legion Beta: Sternfathom's Pet Journal.
This is a treasure that you can find in Azsuna. Loot the treasure chest next to a speared gnoll's corpse (guarded by a naga named Slavehunter Zsanesh) and you'll find a shiny new...toy?
The toy allows you to summon Shipwrecked Captive, a Grand Master Pet Tamer, who offers a Pet Battle account-wide weekly quest. (Only works in Azsuna.) Completing the quest rewards 4 Pet Charms.
After some testing, I've found that other players cannot pick up or complete the quest from the NPC that you summon, only the original summoner can do so. So we'll each need our own Sternfathom's Pet Journals for the new Pet Battle weekly in Legion.
The Shipwrecked Captive has 3 epic (purple) level 25 pets.
- Scuttles - 1570 HP, 315 Power, 294 Speed
- Clamps - 1657 HP, 320 Power, 271 Speed
- Kiazor the Destroyer - 1657 HP, 276 Power, 315 Speed
When I entered battle with him, two of his pets (Scuttles and Kiazor) only had 2 abilities listed in their tooltips. This might simply be a bug.
The NPC will die/despawn (can a ghost die? haha) after a few minutes, but thankfully you can keep resummoning him (the toy has a short cooldown time of 2 minutes). You can only pick up the quest once (per week) though.
If his time runs out while you're mid-battle, he will not despawn until you've left the encounter. No cheating Mister Grand Master Tamer. Can't run away mid-battle just because I'm about to win! :P
What happens once you've completed his quest and turn it in? I'm not entirely sure since I have yet to try out the battle for myself. Will we be able to keep summoning him? Will he be willing to battle us even if we do not have the quest? My guess for the first question is yes. The second question...that's more up in the air.
If the NPC continues to offer to battle even after we've completed and turned his quest for the week, this could mean an endless source of pet experience. Trainer battles are known for their high pet exp yield, especially the ones with 3 epic quality level 25 pets, so...a bit of an exploit there?
With that in mind, my best guess is that no, we will not be able to continue battling him post-quest completion. At least, not until we've picked up the quest again. We might be able to interact with him and he'll have some quirky dialogue for us, but that will probably be it.
Final thoughts: This quest is almost as if the WoD weekly and the 'traveling trainer' (Erris/Kura) were combined. Except there's only one objective to complete for the Legion quest (defeat the Grand Master Pet Tamer once a week), and the appearance of the Tamer is not random. You control when and where (in Azsuna) you decide to battle him. It's like a simplified version of the two aforementioned WoD Pet Battle quest mechanics that's much more convenient for players.
What a fun and interesting find! More unique/different mechanics for Pet Battle content is always welcome. It breaks up the monotony and keeps us on our toes. 'Where will the next unlock/discovery/content come from today?!' :)