Hi. What is the way to make one NPC in the game to fight another NPC? I want them both to be unkillable and just fight each other forever.

Make them both essential, and in factions that are enemies of each other.
*EDIT: You could also add an ability to both of them that constant heals them, this will prevent them from bleeding out. Additionally, you could give them weapons that do no damage.

What faction is needed for it? I have made a relationship of them both that they are enemies. Do I need to create a new faction for it? if yes, what do I suppose to put in it?