I'm planning on reusing one of my favourite old Traveller adventures, Shadows. [DA01 Shadows to be precise]
For campaign play, no xenos required, I should be able to terrify them enough without.
For cinematic add xenos for more carnage.
Short version: players ship is investigating an anomaly on an unihabitated planet with corrosive atmosphere. While overflying, players ship is forced down by an energy beam/blast from anomaly. They can't take off with the likelihood of being shot down and can't stay on ground as there are earth tremors that are getting worse and may disable ship., so have to investigate. They find an ancient looking pyramid structure (including statues of reptile like humanoids) that has been damaged in tremors.
Entering the pyramid is when the fun begins. Interior is filled with circular corridoors with false floors over liquid/mists. There is an animal species living in the pyramid that subsists by eating it's own young, like the Cannibal Frog of Africa. Eggs are laid in the lquid and hatch into snakelike vegetarian juveniles that grow to about 1m long/3kg weight. Then they hibernate and metamorphosize into either a 4 legged carnivourius male of about 6kg or a carnivorous female similar to a large bat 1.2m wingspan. [Can we see why these might terrify the players, as they swim/scuttle/fly about in the misty waters or over their heads in the darkness?]
Eventualy they will discover a control room for a reactor that is venting energy due to damage from the tremors. They will have to work out how to use the alien controls to stop the venting, which will alllow them to take off safely. However this will also cause the reactor to overload and meltdown/explode destroting all evidence of said vanished alien culture.