Well, what is a Marilith? It's a large snake tail with some chassis at the front that holds a hole bunch of arms to fight with - let's go with this basic idea. Snake tail, okay, but let's top it with something scarier then a naked woman with tits (and some extra limbs). What's scary and has a lot of arms? Spiders. So it's a large (I guess maybe small horse-sized) spider with a long, heavy, ponderous snake tail where its abdomen would be.
It won't be waving around scimitars with its hairy spider legs, so let's give it a different method of attack. It bites and poisons, sure, but something else as well. It's half snake, and there's such a thing as a spitting snake, so I'll make it spit... but what? Poison is passe. It should spit something related to snakes or spider... flies! It spits living flies - or more accurately, tiny, vaguely humanoid fly-men. It creates them from the juice sucked out of its victims. The flies don't attack directly. They, ermm... they're flied, they're dirty, they spit on food. They land on food and infect it with the spirit of distrust, intolerance and hatred. They also so small theylook like normal flies at a glance, which works just fine: I want demons to be about more than just hitting hard and being tough to kill. So the spidersnake demon hides in caves, ruins and the like near populated areas and sends forth its tiny minions to spread hatred and chaos via the infected food.
So far, so good, but it needs something more, like some creepy extra body parts. Have you ever seen up-close photos of spiders? Those extra eyes sure look creepy, so I'll work with that. When it feels threatened (say, by adventurers), smaller snakes grow out of its eyes and try to bite its enemies. But here's a great opportunity to add something even more: those smaller snakes (well, normal-sized snakes) also have the forebodies of spiders! They're smaller copies of the entire demon. And since they're copies, they also have a whole bunch of eyes, so you have even smaller growing out of those, and of course they too have a spider body. And eyes. With snakes growing out of them.
As it were, this here is a fractal monsters which has smaller copies of itself growing out of its eyes, and out of the eyes of those, and so on ad infinitum. A single spidersnake is in fact an infinite number of ever-regressing spidersnakes
The Deepest Planter of Ebony Seeds*
4 Hit Dice. AC 14. Reduces all physical damage by 3 HP. Bite attack 1d6 HP + save vs. poison or temporarily lose 1d6 points of DEX. 4 additional eye-snake attacks when not surprised, each as a 2HD monster that does 1d6-1 HP. Does the fly-man food infection thing and also sees and hears through them.
*All stats for World of Zu, the numbers would be completely different for standard D&D.
Next time I might make a monster that combines a bear with a stone wall.