As an adult, Skin of Evil was kind of campy, but not without its charms. As a 6 year old, the episode scared the crap out of me. A demonic black oil monster (nevermind that it could barely move in humanoid form) with a laundry list:
-mentally torture Troi
-suck in Riker
-impervious to phaser fire (this is a big deal to six year olds. Phasers are cool).
-could move around in oil slick form
-could block transporter beams
Killing Yar was almost like the least of those worries; it was very much a display of his power, but I knew someone always dies during these things. But the moment that sold me was when Riker's face popped up in the oil slick and then sunk back down in despair.