I have been playing the River Berbha raid to get Lugh's Myth Clue. I believe the clue is at Wicker Grove, however I left the grove after collecting all the items apart from the Myth Clue. The game has now marked this raid as complete and I cannot enter the location again as a raid. I believe to get the item I need to be on a raid as I need another Viking to help me break down a door, so effectively it looks like this can never be collected and I cannot progress.
Hey there @philwren999, @dech2410 and @Ancient_F! I am sorry to hear that all of you have come across this issue with the River Berbha raid whereby you missed the Lugh's Myth clue, and now are unable to obtain it due to not being able to raid the location again to have assistance with breaking down the door blocking your way.
As @Kormac67 has kindly pointed out, you should be able to reset the location and raid it again in order to obtain the clue if you raid some of the other previous rivers again (i.e. the River Exe, the River Severn or the River Dee). Could you please let me know if this works for you all? 
Let me know as well if you require any further assistance in the meantime.
As far as I am aware, @dech2410, entering the location won't be enough. You need to complete the previous raids again fully in order to progress towards the reset, as it's a process. There's no set number of raids that will be needed, so I cannot advise on that, but if you complete a few and find that the River Berbha has not reset, please let me know and I'll consult the Assassin's Creed Valhalla team again for further information.
@dech2410 Please keep us on this thread posted! I hope it works for you. 
@Ancient_F It's definitely a frustrating situation! I'm glad to hear that you were able to reset the location, however. Thanks for letting us know!
Thanks for the updates, @dech2410 & @philwren999! I'm glad to hear that you've been able to collect the clue from Wicker Grove following a raid reset 
To my knowledge, this is working as intended. After you've raided a location during a River Raid, it will be unavailable to raid again and will show as destroyed. After you have taken part in a couple more raids (by entering a river and then returning to the settlement), the location will "reset" and can be raided again!
If you encounter any further trouble with the River Raids, please don't hesitate to reach out.
Thanks! 
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