My iPhone x/10 keeps dying at 15-20% what may be the problem?

Well, as already said, it is about reaching the critical point of its life (generically indicated at 80%, but I've seen unpredictable barttery's behaviors even at 85/86%, in specific enviromental conditions changes from hot to cold and viceversa).

I recommend that you start creating a full backup of your iPhone X on a regular basis, in the unfortunate event that the battery should suddenly cease to function, so that your data, media, apps, documents stay safe.

These are the prices valid in US Apple Stores for battery replacement (tax excluded, work included):--> iPhone Battery Replacement - Official Apple Support

If you live outside the USA, check those prices in your country.

Regards and best of luck.

Giulio

Your battery might be reaching (or have reached) the critical point of its life.

What's the maximum capacity percentage of your battery in Settings>Battery>Battery Health?

Regards

Giulio

My pleasure :)