Garthanos
Arcadian Knight
In game, there is no meaningful mechanical difference between Arthur's power coming from Excalibur or Arthur's power actually being inherent while he believes it comes from Excalibur. The character is fully capable of chalking up his good performance while wielding a different blade to divine providence, if it ever comes up.
Hmm potentially making Arthurs power no more or less intrinsic than Lancelots? You could certainly envision an Arthur where any weapon he might wield even becomes... the Dragons tooth.... that might be an interesting take on it.