Pool Cleaner

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Hopefully, this Beta issue will be fixed before launch. ***Didn't happen, so... DO NOT SEND ALYZA BACK TO THE ORPHANAGE! ***

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When you've completed the An Orphan's Request story quest, [npc=215254] will be around when you're in the Hungering Pool area in Hallowfall. e.g. while you're on this Pool Cleaner WQ.

When I used her gossip option to send her back to the orphanage, the hidden spell [spell=446876] flagged a hidden quest [quest=82131] complete for my Warband. Usually, this should only determine if I have Alyza with me or if she's waiting in the orphanage until I tell her to join me again (in the Hungering Pool).
But in Beta, all surrounding Kobyss and almost every creature/NPC in the zone disappeared, and that wasn't all! Every Warband character who went to Hallowfall was stranded in an almost empty zone with no hostile creatures, Keyflames, Schools of Fish, or even friendly creatures like Flightmasters.

It took me two weeks, the confirmation by other affected players, and the help of Matjz to figure out the issue and a potential fix (unfortunately, nothing came from Blizzard's side during that time).

1. When you're affected, head back to your empty Hallowfall, and walk up the stairs in Mereldar's orphanage building.
2. 
   
3. Other orphans are probably missing, but you should see [npc=219368] up there. Talk to her and use the "You can join me in the Hungering Pool again." gossip option.

That's it! Internally, [spell=446905] will flag that hidden quest incomplete for your Warband and bring back all the missing things! Good luck!
/way #2215 41.79 55.78 Alyza Bowblaze

When you pick up this quest and Alyza appears next to you, DO NOT tell her to go back to the orphanage.

It'll set you in a different phase and pretty much every mob throughout Hallowfall will disappear. You can find her in the orphanage and ask her to come with you to the pools to fix it however.

So apparently if you want to get ahead in WoW you just need to help traumatized children indulge their unhealthy bloodlust.

The mobs are not in the sinkhole.

After following all the advice in the comments, completing all the An Orphan's Dilemma quests and ensuring Alyza Bowblaze was with me, I still couldn’t find the mobs.
My mistake was flying down into the sinkhole, where also the entrance to the "The Sinkhole" delve entrance is - don’t do that! Instead, stay around the entrance of the sinkhole above and look for Hungering Slicer. These are the mobs you need to kill.