Unique artifacts are magic items which become accessible at Construction level 8.
If the unit carrying an artifact is killed in battle, artifacts will always be picked if possible. If an artifact is lost for whatever reason then it can be reforged.
If an artifact is in existence then it will become greyed out on the forge magic item selection screen. If more than one nation attempts to forge an artifact in a single turn, then one nation will randomly succeed while the others will lose the gems spent on trying to create the artifact.