Hearthpwn

this deck was made before wing 3 came out so it's kind of out dated. it still does well but since the meta changed to aggro shaman it's been rough. but mulligans depend on the match up. in general always keep poison, cold blood, egg, raptor and leaper gnome. keep abusive and deckhand vs paladin, hunter, and warlock or vs everything if you have coin egg into one drop cold blood. keep coldlight and shredder vs warrior. if you know the mage is freeze and the warlock is slow like reno hand lock or maly, keep cold light to mill them. keep evic vs tempo mage, hunter, zoo or maly loc. usually you don't keep sap but if you already have a good hand, keep vs secret pali and druid. i might have missed a little but that should be fine

do that with rogue and prep. throw in a cold blood lol

This card is bad in mill rogue for many reasons.

1- decreases the consistency to an already inconsistent deck. adding more cards that do nothing immediately when played and doesn't help with getting cold light, surviving, or thinning your opponents deck (the opposite actually).

2- hinders the decks' main win condition.  sure adding cards to their deck doesn't matter if when they draw it and it cycles but the whole point it to make them over draw to run out of resources against you so you're coldlights have no positive effect to the opponent with 10 cards in their hand. if you make them overdraw ambushes, you're basically giving them a chance to draw better cards and you are less likely to get them to fatigue beacuase you added the abushes in the their deck while contracting the benefit of deathlord. the card it self could be an alternate win condition but sacrificing the consistency of the main one for an inconsistent one is redundant. i can only see it being played after the opponent is already in fatigue or don't have enough cards in hand to mill it to guarantee the full effect goes off for 3 4/4's in one turn. (which i agree is viable but very situational meaning it has to sit in your hand the whole game. i actually want to see this happen so much) but triple coldlight lethal is enough and you can't even do that if your board is full. let's say you get 6 4/4's that's 24atk but you have to get through taunts and might pop an ice barrier +1 damage through fatigue when they start thier turn. but that requires set up and you usually want to use your mana that late in the game just to survive and actually get them to fatigue.  3 cold light during fatigue is 21 damage + 7 when they start their turn so 28 total and that's with having no board. 

3- you don't wan't to vanish/shadow step your 4/4s. every other minion has sub synergy with cards like vanish and shadow step (other than thalnos). even returning a deathlord making it full health with taunt is more valuable as it has more possible health gain or take off a silence. you usually don't want to fill up your board either as it can make you overdraw with vanish.

4- you're usually not looking to be aggressive. 4/4's are versatile bodies for trading but mill has very efficient removal as it is. that's why cards like dancing swords and mukla aren't played in mill anymore other than just plain back firing which this card is capable of doing as well. on the other hand those cards as well as this card can fit in a more aggressive style deck not looking to fatigue but to go face to take advantage of the big-ish bodies. (in theory. they might still be bad)