So as was originally stated on the Known Issues List

• Avenge quests are nowhere to be found (which has sense been updated to state "Avenge quests are difficult to spot"). This probably needs to be updated to reflect that Avenge Quests spawn are extremely rare and only occur after extensive idle time in specific zones

When the players were faced with a challenge we thought was impossible we decided to come together because of the rare reports that some people were in fact getting an avenge quest but only one and never any more. I decided to start a thread and pull in as many people as possible from the Ubi forms, Ubi AC Discord, and Reddit users. We went through 6 grueling days of trial and error and death and death and death. Did I say death? During our test we limited ourselves solely to East Anglia (between Ruined Tower spawn point, Sunken Glory, south of Sunken Glory at the bandit camp, and Scotthoh Farm; also see reddit post for map). The reason for this is because these were where the sporadic rare reports came in from originally. Ultimately this is what we found out which nearly everyone agrees should be changed as we don't believe this was the intended design implementation:

• Avenge bodies will only spawn when staying within a small radius area for an extended period of time.
• Average time for a body to spawn seems to range between 5-20 minutes IDLE in an area BUT even then sometimes the bodies don't spawn. (Example video from start to finish for avenge quest at the "3-boar site" east of the Ruined Tower spawn:
  ) (6 minute, 40 second wait in this case)
• It would appear that bodies are pre-designed to only spawn in specific areas. We've currently only identified 3 in East Anglia.
• We believe that once an avenge quest has been completed there is a cooldown period (possibly 60 minutes) before another avenge can spawn in that particular area.

Here is a step-by-step guide I created on a reddit post to help others achieve the Timed challenge and for a few of us completion of the Core Challenge Hall of the Slain (Completing 10 avenge quests):
https://www.reddit.com/r/assassinscreed/comments/l4sp0p/ac_valhalla_how_to_avenge_a_player_as_of_the/

Recommended Fix: Ideally no one should have to wait 5-20 minutes idling for a body to spawn an avenge quest. If there is a spawn timer for this duration it should only be 20 seconds upon entering a location. However, the cooldown could remain at 60 minutes if that is indeed what it is. I would also recommend reviewing the potential spawn points for avenge quest to ensure their are proportionately located throughout all regions.

Please let me know if you have any other questions or concerns. We tested this pretty extensively to try and ensure we didn't miss out on a challenge reward but we know there are likely more aspects to this issue.