Marcus was no Gray Rock Hyena, a low-level demon; he was indeed a level one Awakened, and the energy within him was definitely worth Duke's absorption.
If Duke did not absorb the energy from Marcus, it would seem quite unreasonable.
So he had to perform this act in front of the caravan. In fact, even when Finn and the other person saw it, they found it quite normal and did not think in any other direction.
They could never imagine that this scene was deliberately staged by Duke, nor did they think Duke was someone from the Other Plane.
As for Marcus's body, people simply thought it was completely crushed by Duke's Yellow Sand Giant Claw and mixed with the sand, leaving only some blood on the ground.
The once tense canyon became exceedingly quiet, as if everything was merely an illusion.
But everyone knew this was not an illusion; the figure standing before them, Master Jelot, was an exceptionally powerful level one Awakened.
