In the Classic seventh Doctor comic A Cold Day In Hell, The Doctor and Frobisher encounter a dreilyn - also known as a heat vampire. Dreilyns survive by siphoning body heat from other lifeforms. Usually only small amounts that go unnoticed, but in this story She was starved due to extreme cold and was leeching lethal amounts.
I put a Dreilyn in as a prisoner on the Judoon ship. On the way back from the engine room to the bridge, she is an escaped prisoner. Easy enough to rationalise her intense hunger - perhaps she's been starved for contact in her cell, or perhaps the heating is broken on the detention level.
The dreilyn works great, because it gives the players a chance to overcome a threat via thinking and talking - simply convince her to let them take her to the engine room, which is intensely overheating. The Doctor or other worldly character might recognise her species, otherwise throw in a hint where she says "I must have your heat" or something of the likes.
My plan was to have her (inadvertently) save them from an escaped raxacoricofallapatorian who had them cornered. She sucks the monster dry of heat just in time, but still needs more. The players are hard pressed to fight her or run away, so they'll have to use their heads. My players had already accidentally released the raxacoricofallapatorian, so I had to improvise a little.
Nothing brilliant or profound, but it gives you a cool encounter. Cheers.
I'd only half stated the drielyn for my campaign. I'd only had the game for one week (during which I worked a lot) Uder pressure from my GF I ran Judoom having only read most of the rules. I only had a couple hours to familiarise myself with the adventure and decide to add a dreilyn.
Anyway, I stated this up and added it to the alien write up section (there's a link added to the first post). I'm new to DWAITAS, having only picked it up two weeks ago and still not quite read every page of the rules. But I did my best - any GM knows full well they can modify it to their liking.