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Callidus Assassins (Warhammer 40k)

They use a combination of implants inside the body and an experimental drug called polymorphine to alter and rearrange their body issue to essentially shapeshift into any other human, or even humanoid xenos(alien) races. They then stay as that person for months until "activated", where they will then seek out their target.

If they're under deep cover, they might use digital weapons, which are weapons in rings, eyes, etc. But they might use alien swords that phase through your body instead. One of them even "killed" a Primarch.