Delusional Heartbeat. The cursed arm.
It is the arm of the elemental of malice, Shayṭān, and excels at cursing humans to death.
Using an ether lump, it creates a dual existence from the reflected image of the assassination target.
By crushing this imposter resonating with the original, it curses the target to death without ever laying a single finger upon them.
It is a convoluted method of killing, but one well-suited for assassinations.
For instance, even if the target was wearing armor with a defense score of 1000, Zabaniya would grab hold of the false heart from above the armor and deal direct internal damage.
Somehow, it feels a little bit like an illusionist's trick.

Fate/side material: Fate Encyclopedia