how to enter the ghost swamp cave?

I’m in the ghost swamps NE of Fantrima. I think in the caves there is General Decimus to be rescued. But I can’t find him. Not in the old tomb in the NW corner of the swamp, anyway and the only other entrance to what might be a cave, located pretty much in the very middle of the swamp, is blocked with stones and rubble. I can’t enter. On the map this rubble looks like a cave entrance. Do I lack a skill or tool? It almost looks like a bug, though.
Thanks for some clues and hints

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Try talking to the guy who send you on the mission. I haven't found the cave yet, but he (the guy beneath Fantrima) says that he will dispatch 2 men to remove the rubble.

Yep, talk to the knight who trains you, Septemris, and he will send two men to clear the rubble. You can then return to the screen where you said you saw the blocked cave, it will now be clear for you to enter. In you go

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Thanks mates,
worked well, I got the General, the divine lamp, I'm a squire meanwhile, heading off to warn the Dernath guards and look forward to what'up next. See you soon with the next trouble

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