Anyone knows what to if i draw ungoliant's spawn for shadow effect, and i got the path where ungoliant's spawn is the goal of the quest? Should i use it's shadows effect, discard it and then wait for it to come back a the top of the encounter deck again?

Yes, that's what needs to be done.

Note however that when you start quest 3B, each player gets the opportunity to bring a spider into play from the encounter deck or the discard pile. So, that's your occasion to bring Ungoliant's Spawn into play (even if it was previously seen as a shadow card).

If you don't want to do it at this moment, then yes, you're at risk of seeing it pass as a shadow card and then having to wait until you've gone through all of the encounter deck.