how come King of Swords achievement have only 5% of players?

[Nier Jendt]

Is it a rare ending or what?

[Zebedee]

Steam still counts DLC achievements as a percentage of all players, doesn't it? So first you need to own the DLC, then you need to complete. All the DLC ending achievements are going to be relatively rare.

[Tokenn]

AS well as what Zebedee mentions, I think the combination of decisions you have to make to get there would be less natural than the other endings. I deliberately went through all the decisions to see what happens...oddly, King of Swords was the first ending I got.