OK, technically this image is from Final Fantasy, but you get the point. What environmental factors could cause this to evolve? What would the starter species be like?
At least in non segmented animals like vertebrates, duplication of limbs is more or less out of the question. First of all, a single chance mutation has to create a pair of limbs which just happen to not be in such a place as to interfere with any other organs or anatomical structures. Then, that mutation has to be somehow so advantageous that it takes over the population. That's not going to happen, realistically.
So, mariliths will have to be non-tetrapods. They could be from a parallel lineage of land vertebrates, but I have a different suggestion; giant insects.
Above is the larva of a water beetle. It has six limbs and a serpentine abdomen - just like a marilith. Suppose that, in an oxygen rich world, a neotenic form of water beetle evolves giant size. It would be an ambush predator, with binocular vision - hence a vaguely humanoid face - and stuff huge raptorial limbs. It would lie in wait on the water's edge, and, when a prey item came near, rear up in a pose much like the typical naga posture.
It would be a very long and unlikely road. Begin some fifty million years ago with a random mutation in the HOX genes of a small primate, making it from a four into a six-limbed creature. That's the easy part, once the unlikeliness of the mutation and its survival goes through (tetrapods actually emerged way earlier than that - snakes are tetrapods themselves, but I expect you don't want a planet-wide shared hexapod bauplan).
From there, environmental pressure to drive it to water and make it amphibious, and develop its tail into a swimming aid seems the best way to get a large tail. Then back to earth, with the tail as locomotion - the terrain must be such that the tail is a huge competitive advantage. The coexistence of a human-like upper body and a scaled lower body is the hardest part; I'd actually expect the whole of the skin to go scaly, or more likely thick, leathery and with horny protrusions.
- They start out mammalian vertebrates because otherwise they could not be blonde.
- That second pair of arms is the pair we use for legs. She has that tail for locomotions so she can use them as arms too.
Now we have all 4 tetrapod appendages accounted for. Hmm...
- Third pair of arms is from a conjoined twin. She is wearing that big hat right now so you can't see her head, but the ball on top signifies respect to the twin. At home she lets the twin head look over her shoulder and kibbitz about her cooking. The two arms controlled by the twin are less predictable than the main 4.