When you cast a spell, you can choose to infuse it with vital, living essence (this must be declared at the moment of casting). Doing so doubles your proficiency bonus to the spell hit/save DC of the spell, as well as the additional effects. Vital empowerment can be performed one of two ways.

Self: you sacrifice your own vital essence to infuse into the power of your spell. To do so, choose a number of your own hit dice to roll and add your spell-casting modifier to determine the damage dealt. Your maximum HP is reduced by the same number rolled on the dice minus your spell-casting modifier. This also reduces the number of hit dice available to roll during a short rest, and you cannot restore your maximum HP until completing a long rest.

Synergy: you touch a willing target to surrender its vital energy to your spell. Utilizing its hit dice plus your spell-casting modifier, you calculate the damage dealt. Otherwise, it follows the same rules as utilizing your own energy, with your subject's maximum HP and hit dice reduced until they complete a long rest.

Sacrifice: You touch a target that must be paralyzed, incapacitated, or restrained and engage in a contested challenge. The contest is determined by the creature, a commoner would utilize their base CON, a fairy might use its CHA. You will utilize your spell-casting modifier. On a victory, you can draw up to 1d6 of their hit-dice to use in your casting. On a failure, you take 1d6 force damage and are pushed 10 feet away from the target.