Lifeleech

Is a small mod that adds the titular Lifeleech mechanic! 

Killing enemies applies a short-lived Health Boost effect to you. The more enemies you kill in succession, the higher your Health Boost will rise!

The config is detailed, so you can set it up however you like:

- Enable/disable the mod
- Whether or not the mechanic works even without the potion effect active (It does by default)
- Mob Whitelist tag for you to hand-pick what mobs have their health stolen on death (Disabled by default)
- Mob Blacklist tag, so you can prevent easily-farmable mobs from being used to potentially exploit the mechanic (Disabled by default)
  - If neither of these are enabled, the mechanic works on all mobs indiscriminately
- How long the first Health Boost lasts for
- How long the additional levels of Health Boost last for
- Max additional levels of Health Boost (4 by default, for 5 total levels/10 extra hearts)
- How long until more Health Boost levels can be earnt from killing via the Satiated effect (Added as a buffer to prevent sweeping attacks granting multiple Health Boosts at once - 1 second by default)
- Whether or not the Health Boost effect decays gradually or vanishes all at once upon it's timer running out
  - How many levels of Health Boost are lost at the end of each timeout period

There's also an mcfunction you can edit that fires every time a mob is killed with the mechanic in play.

Maybe you should edit it so that the player is rewarded with regeneration or saturation upon slaying enemies, else they'll have to rely on their own food items or other potions to make use of those extra hearts earnt ;)

Designed with the idea of a vampire-themed modpack in mind, but could also be useful for those trying to add extra incentive to engage in combat in their packs. 

Hope you enjoy it! :D