15th-level Echo Knight feature
You've learned to absorb the fleeting magic of your echo. When an echo of yours is destroyed by taking damage, you can gain a number of temporary hit points equal to 2d6 + your Constitution modifier, provided you don't already have temporary hit points.
You can use this feature a number of times equal to your Constitution modifier (a minimum of once). You regain all expended uses when you finish a long rest.