I dunno it was sorta Grievous-esque but also unique. Looked as if it uses others bodily fluids to fuel itself (judging by the similar tubes all over it, which is pretty grim.I don't know if it's hand-waving, but I distinctly got the impression that the first thing that happened when Mando was injected was total body paralysis. He would still need to move something in order to set off the whistling birds or flamethrower or grapple cord, but he couldn't cause he was totally incapacitated. That explanation worked for me.
Then the eyeball thing started to drain his blood. Gross. That eyeball was a cool effect but didn't match anything we've ever seen in SW so far.