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.
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.