What happened to my iPhone X battery?

Your iPhone X probably needs a new battery.

Batteries degrade over time with usage due to chemical aging, that's just how they work.

You can expect about a 1% drop per month with average use on an iPhone. This is an approximation, that is why it may be plus or minus a few percent on a month to month basis.

Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.

iPhone Battery Replacement.

iPhone Battery Repair & Replacement - Apple Support

About Lithium Ion Batteries

iPhone battery and performance - Apple Support (IN)

Though you have not explained your battery problem, I will make a guess that as yours is an older phone, the battery probably needs replacing. The typical Apple phone lasts about 2 years with normal usage. Yours is older and the battery is probably dead or dying. You need to make an appointment at the nearest Apple Store for possible Battery Replacement....

Apple Store - Find a Store - Apple