If you talk to King Arthur after receiving the Quest from him, he will tell you to seek the blessing of the Lady of the Lake. (Or just have him tell you this as part of giving you the quest.) After this, your next fountain dip is guaranteed to produce Excalibur, if it does not already exist.

You can get some rare intrinsics by eating artifacts that confer them. For instance, eating Excalibur could give level-drain resistance, or eating Grayswandir could give hallucination resistance.

The chance would be much less than 100%, possibly even lower than the standard odds of getting an intrinsic from eating jewelry.

You can’t dip successfully for Excalibur if water nymphs are genocided (and maybe extinct, as well). Not because you killed the Lady of the Lake, but because she’s mad at you.

Only knights of XL 7 or higher can dip for Excalibur; but Excalibur is available as a lawful sacrifice gift.

Excalibur starts out much weaker than it currently does, but you can perform further feats to enhance its power and give it more abilities as the game progresses. Such feats include: slaying a dragon, returning to King Arthur having finished the Knight quest, getting crowned as a lawful, re-dipping it into a fountain at a high experience level.

If you dip for Excalibur while using a helm of opposite alignment to be lawful and the attempt would succeed, the Lady of the Lake notices that something is wrong. She emerges from the fountain, grabs your sword, turns it into Excalibur, and rips off your helmet. This allows non-lawfuls to get Excalibur, but only by defeating her.

Raise the XL threshold of dipping Excalibur for non-Knight roles to around 9-12. Knights can still dip at level 5.

Samurai can’t use Excalibur, because it’s cross-culture. The Tsurugi should be buffed to be quite good enough.