Hearthpwn

I found it in a pack, and was utterly disappointed at first. But then I decided to give it a try using a murloc list.
Well, I was surprised by how effective is in this deck! I would say it is, mostly becayse its effect is unique and you cannot truly replace it: put it into the battlefield in the early game and, if your opponent doesn't deal with it, it can win you the game the turn after. Moreover, this is the only card (except from the combo Leeroy+Windfury) that can help you in the midgame if you fail to defeat your opponent or you lose your board before turn 6-7, when paired with Underbelly Angler.

I understand that Scargil is a pretty niche legendary, and if you are f2p (as well as I am) I am not sure this will be the best legendary to craft (quite the opposite). Maybe you can replace it with Old Murk-Eye if you have it (if you want a more aggressive list) orEveryfin is Awesome to stabilize the board since you will not have any comeback chances. 
But if you like Murlocs and play wild a lot, this could be one of the best minions for a long time (and, also, its power level grows everytime a powerful murloc is released, even though it's a rare eventuality).

tl;dr: it's one of the core cards of this deck, so if you have tons of dust to spare craft it. If not, I would suggest replacing it with a cheaper alternative (Old Murk-Eye sine it was given for free once you collected all the murlocs in the basic and classic sets, Everyfin is Awesome)