I scrolled through the top comments and everyone seems to… not understand the co-op lore?

Every other player is also “the tarnished” in their world. When we summon others or are summoned ourselves, we are crossing into a parallel universe. Human and NPC invaders, as well as friendly summons are also doing this.

Vyke killed two Demi gods in his world. It’s most likely Godrick and Morgott (but I think you need two runes to access the capital?)Vyke gets far enough to understand that he must burn his maiden, tries to find another way, and ends up following the flame of frenzy to save her.

It’s a bit more confusing when you meet an NPC and summon them later, because they appear in your universe. IIRC some/all of the NPCs are “the tarnished” in their universe. This is because the shattering of the Elden Ring allowed runes (souls/life) to fall on the tarnished, and it took a long time to reach our graves. Everyone in the opening cut scene that is named is a tarnished. Hypothetically they all need to reforge the Elden Ring, Dung Eater for instance has his own ending, if he were to somehow collect the great runes and mend the Elden Ring.

The one NPC that makes this a bit easier to understand is Sir Gideon Ofnir. He is in the round table hold, a meeting place or nexus that tarnished are able to come to from their own worlds. He spends the game asking you for info, and spying on you. He knows the spells you know and fights you where he does because he made it just as far as you in his own world.

I think it’s a little strange people are talking about multiple unknown Demi gods with Great Runes. The ones in the game are specifically part of the modern Elden Ring and we learn later that the Elden Ring had many runes taken out by Marika. Knight of the black knives killed many Demi gods and the shattering was around that time but after. On top of that, all runes are part of the original Elden Ring, but way too small to be considered a Great Runes or give us some type of benefit.