it seems at least that only the bottom arms can regrow or he suffered a injury to his top arms that made them imposible to regrow. but only difference between each enemy aka vandals dregs captains ect is 1. how many arms they have and 2. the amount of ether ration they get. the more ether they get the bigger they become.
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If you read the grimoire closely they have to go through a ritual to regrow their limbs, I do not believe they can just do it whenever they want.
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That is interesting for sure.