In God of War Ragnarok, when you first arrive at the King’s Tomb during the The Word of Destiny objective in The Path main story quest line, the only collectible in this small region will be marked as “Undiscovered”. Shortly after arriving, however, you’ll find the sluggish grip of Skofnung, King Hrolf Kraki, and this will turn 10 of the “undiscovered” collectibles scattered throughout the game into slayer tombstones.
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In God of War: Ragnarok, when a collectible changes from Undetected to something specific, it usually means there’s nothing stopping you from collecting it, except maybe a fight or a puzzle or two. This applies to nine of the Berserker Tombstones, although you’re advised to brush up a bit before attempting them as the Berserkers are tough opponents. But it doesn’t apply to the Berserker tombstone in King’s Grave. Confusingly, while this one appears on the map along with everyone else, you can’t get it right away.
How to fix the King’s Grave Berserker Gravestone “Bug”.
Despite what it looks like, there isn’t a bug preventing you from collecting the Berserker Tombstone in King’s Grave. The fact of the matter is that the King’s Tomb Berserker Tombstone doesn’t actually activate until you’ve defeated the berserkers at all of the other nine Berserker Tombstones, but the game doesn’t make that condition very clear at all.
So the way to “fix” the problem is to look for the other nine berserker tombstones, some of which are occupied by more than one berserker, meaning there are 13 berserkers in total. There are berserker tombstones on Alberich Island, Nidavellir and the Jarnsmida Pitmines in Svartalfheim; The Barrens and Forbidden Sands in Alfheim; Pilgrim’s Landing and The Sinkholes at Vanaheim; the Lake of Nine in Midgard; and the fog fields in Niflheim.
https://www.gamepur.com/guides/what-to-do-if-the-kings-grave-berserker-gravestone-is-not-working-in-god-of-war-ragnarok What to do if the King’s Grave Berserker Gravestone doesn’t work in God of War Ragnarok?