The time will come when your descendants won't be able to wield that sword anymore.
The sentence is OK in AmE (in BrE "any more" is two words). In the case where "X when Y" means that X happens at the time that Y happens, we use the present tense for Y even if Y is in the future. For example, we say "I'll explain everything when I see you", not "I'll explain everything when I will see you".
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The sentence is OK in AmE (in BrE "any more" is two words).
In the case where "X when Y" means that X happens at the time that Y happens, we use the present tense for Y even if Y is in the future. For example, we say "I'll explain everything when I see you", not "I'll explain everything when I will see you". However, your "when" is not like this. The sentence does not mean that "the time