Essentially, a fourty-foot rattlesnake with horns that can raise itself like a cobra. But unlike a rattlesnake or cobra it's venomless - because of its size, and thus the size of the food it needs, it couldn't pump enough venom into a victim fast enough to effectively kill it. Instead, it ambushes a victim, holds on with its fangs, curls around it, and impales it with the dagger its rattle evolved into. Malarocs are actually fairly placid, and only attack if they're hungry or annoyed (and malarocs don't annoy easily).
I know, not a very original design. But the Death Adder is an undead one.