How do I add new artifacts?

Try not to get it randomly, but use the Map Editor and a script on the map to give it:

GiveArtefact("MY_HERO_SCRIPT_NAME", SWORD_OF_HELLMARE);

It should work...

Added 1 minute ago
Just hypothetically...

So, at the start of the map, new artifacts won't appear in place of the random artifact boxes? And if the GSC (Game Scripting Community) creates a map, they won't appear there either?

That's a shame. That's not what I was aiming for.