A heartspike is a device that's embedded into the aorta of the person to be controlled or killed. A controller remotely connected to the heartspike can send the intended victim's heart into various levels of pain, including a heart attack or death.
The heartspike can be removed from the heart through surgery.
One was shot into Ephraim ti Horn during his escape attempt from Eagle Rest. It was removed by Electra au Barca and altered into a bomb by Pax Augustus, per Ephraim's request. The altered device was then activated in an attempt to kill Volsung Fá, as well as Xenophon.