There's a skill which makes people disappear and leaves speed lines....

jelliedsoup

I've seen it in PvP but don't know what it is. It looks quite useful when ambushed, does anyone know what it is?

It's not Shadow Cloak...

ksaggies04ub17_ESO2

My best guess is that it's a sorcerer's lightning bolt. Are the lines blue by chance? If so - that's probably what it is. The person doesn't fully disappear, it teleports them 15 or so yards ahead.

Psychic_Kitty

yeah assassin skills wise you could be referring to the shadow path skill....but I don't think so....most shadow skills are not very useful to evade or escape....even the teleport attack has a shorter range then mages and their teleport powers.
Plus you have to remember...night blades only get about 2 seconds of real invisibility time....potions out do us.