Re-posting in the wow classic bug forum. See:
Ran scarlet monastery cathedral with another paladin and turned in “A Brother in Need” to NPC Aeonas. The other paladin completed the next quest, so I left the instance to reset it. I figured I could just pick up the quest and complete it on the next run. After the reset and returning to the room behind the boss, the NPC Aeonas did not spawn for me. I went back to Stormwind and Katherine no longer has any quest options. At this point it seems the quest is bugged out for me and I can’t see any other way to reset it.
Hoping someone at Blizzard will see this and respond. Already submitted a description of the bug in-game. Seems like a bug that would occur in a low frequency of players but is highly impactful given the specific rune ability.
Occurred on WOW season of discover, Wild Growth - US, player Konovalov.
EDIT: Solution Found
Ended up finding another Paladin on the “A Brother in Need” part of the quest chain. After clearing scarlet monastery cathedral in the same group, NPC Aeonas spawned in for me as well. Was then able to complete the quest.
Still a bit weird as the NPC spawning in seems to be triggered by having “A Brother in Need” (though may also have spawned in if I had the other quest in my quest log). Recall other quest related NPC in other instances typically are present whether you have the quest or not.
Would be great for someone at Blizzard to look into this, as it may not be easy to find another person on the same portion of the quest to group with and clear the instance.