“The Unseen Masters of Il'Ignoth saw empathy as a weakness to be purged from their minds, as well as a method to cripple and control lesser beings. The devised spells to harness and connect, to turn their sentient foes’ inbuilt empathic sense against themselves. But their arrogance blinded them to the fact that empathy can be a strength, and when their slaves realized this, it would spell doom for the Masters of Il'Ignoth.”
Empathy: Psychically linking entities so that they mirror damage and healing between each other.
Thrall Empath: A withered husk of a man with arcane machinery implanted into his skull. Hoarse rattling voice pleads “kill me…”
All damage Thrall Empaths take is broadcast to all other creatures within 40 ft as they experience its pain. Any magical healing it experiences is broadcast in the same way. When a Thrall Empath dies, all creatures within 40 ft must make a (lenient) death saving throw as they, too get pulled along with it.
Send one of these guys at the PCs as the first encounter. Let them kill it, freak out at their lost hp, mentally digest its ability, etc. Then at some later time, throw three or four at them. And yes, multiple Thrall Empaths can totally feedback loop off of each other. If your players are smart and realize this, they will immediately enter lateral-thinking-combat-mode, which is where you want them. If they don’t realize this and execute numeric-optimization-combat-mode, the ensuing TPK is on them, not you.
Thrall Empaths can come in all shapes and sizes. Little twisted twitchy ones, big hulking smashy ones, dainty tall spindly ones, limited only by your descriptive power and stat-blocking skills.
Empathic Bond: Two creatures now have a psychic link, and any damage or healing one of them takes gets mirrored onto the other.
Empathic Beacon: One creature now broadcasts the damage or healing it takes to all other creatures within 40 ft.