Whenever Plague Wight becomes blocked, each creature blocking it gets -1/-1 until end of turn.
An ability that triggers when a creature becomes blocked triggers only once if two or more creatures block it.
If Plague Wight leaves the battlefield while its triggered ability is on the stack, the creatures that were blocking it before it left the battlefield get -1/-1.
If all creatures blocking Plague Wight die, Plague Wight remains blocked. It won’t assign any combat damage.