(How to run this: give a saving throw when a PC would suffer harm. Describe what is about to happen to them and ask how they try to avoid it. Have them make the throw. On a success, they avoid it as they said. On a fail, they do suffer the harm: describe it. The monster's "when clauses" interrupt things like attack rolls. If a when clause triggers and it has "or-separated clauses", roll 2d6: 6-, first clause; 7-9: middle clause; 10+, final clause.)
I gather bones into my body, for in time I must molt. All surfaces are floors to me.
When they approach me with violence, I strike, or I move through and past them to tear their flesh and weaken them, or I break apart and let my bones rattle still.
When I strike, I swallow one of them, or spit my blinding poison, or snap off an arm or leg in my jaws.
When I break apart, I reform upon anyone who lingers in my bones, and my ribs will pierce them and rip them apart.
When they try to hide, I discover them suddenly, or make my way noisily to them, or perhaps I am bored and leave.
When they try to run, I strike, or pursue, or perhaps I grow bored and let them.
When they hope I won't notice, I do and I strike, or I do but I don't let on, or I do and don't care.
When they invoke The Lord of Serpents, let them prove their devotion the holy one, and I will lie on my back, open my ribs, and convey them into whatever darkness they wish.