Hearthpwn

theres a video on it on battle.net go watch it it pretty much explains it. 
you discover 3 different times
first time is cost 1 5 or 10 
10 is picked in the video

Second time is first spell effect: 
Options for 10 mana are: 
Powerful mojo > add three cards from your opponent's deck to your hand
Felbloom >  deal 6 damage to all minons
Kingsblood > Draw 3 cards

Felbloom is picked in the video

third Discover is the second effect:
Options for the 10 mana are: 
Ichor of Undeath > resummon 3 friendly minions that died this game
Stonescale Oil > gain 10 armor
Mystic Wool > Transform all minions into 1/1 sheep

Ichor of Undead is chosen in the video.

So your options for 10 mana are: 
Board clear + Resummon
Board clear + Armor
Board Clear + Sheep (which is useless)

Add 3 cards from opponent's deck to hand + Resummon
Add 3 cards from opponent's deck to hand + Armor
Add 3 cards from opponent's deck to hand + Sheep

Draw 3 cards from your deck + Resummon
Draw 3 cards from your deck + Armor
Draw 3 cards from your deck + Sheep

Depending on when you play him in the match will greatly decide what is useful for you. 
the 10 mana Board clear means nothing if you're ahead all game. 
the 10 mana draw 3 means little if you're in a top deck mode and behind (delays you 1 turn from playing any draws and allows you're opponent to farther their lead.) 
the 10 mana copy 3 cards means a gamble if you're in a top deck mode. (stealing 3 copies of 2 or less drops from you're opponent doesn't help if he can clear them as fast as you play them.
the 10 mana Resummon can help to regain tempo but you risk paying 10 mana for say 6 mana worth of minions. (3 2 mana costs)
the 10 mana armor isn't incredibly useful unless you're trying to push another turn (if you're behind, it could say one turn, if you're ahead there was little reason to pick it to begin with.) 
the 10 mana Sheep is useful if you're opponent is playing a synergy style deck (bascially if you're opponent is playing anything that brings cards back N'zoth, Anyfin...etc or anything that buffs similar Pirates, Murlocs, Or has a lot of stronger than 6 health minions (because you can combo it with the deal 6 to all.) 

A couple of things to note.
1) the spell maybe a bit more random. there is nothing stating that these 6 options only appear on their stages on the video. 
meaning: It maybe possible to pick Sheep + Resummon because it might pick 3 effects from a list of say 8 (with 2 unrevealed) and then pick 3 effects from the remaining 5 (meaning that you may have games where he doesn't give you the option of Felbloom.) 
2) the Spell will activate in the order the effects are listed. Meaning Felbloom + Sheep would deal 6 damage to all minions AND then transform all surviving minions into sheep 
3) the mana cost options may also be from a list of more than just 1 5 or 10. (though it seems like that's unlikely. since 1 would be asking for a quick spell, 5 would be asking for a midgame spell, and 10 would be a major spell) 

I hypothesis that all mana cost versions will be just weaker versions of the other options.
Meaning that a 1 mana cost would probably give you options like: 
Felbloom > deal 1 to all minions
Powerful Mojo > Steal 1 card
Kingsblood > Draw 1 card
Ichor > resummon 1 
Oil > 2 armor 
Wool > Polymorph 1 minion 

this would still make an overpowered spell (which any 1 mana cost spell with 2 effects would be anyways.) 

Its possible that 1 mana cost spells will also have Negative effects like take 2 damage in addition to the other effects. but hey this is just a theory...a game theory.

beneath will be there until spring. so it'll make a common appearence if players find a way to make reno decks more competitive.. Right now mage seems like the best fit, since you can use Discover and spell generating cards to fill in duplicate slots (babbling book, cabalist tome

I'm excited as Fuck. I saw that image and verbally shouted WOO

It's not impossible to play around you play around what it becomes not it self. Meaning each turn you decide to play around what it changes to. For instance if it's a card worth playing you'll use it, if it's not you'll ignore it. If it's worth playing it may change how you would play your turn (getting a battlecry minion may be better than playing a spell of the same cost from time to time. Getting an acolyte of pain could be useful for more draw .. Basically you add him in hopes of getting value out of him, rather it's a battlecry, a decent body, or a tech card. He's not to replace anything, but to be used in addition to. Like if you use cthun and get ANY of cthun or mistcaller you got value. If you play yogg and get any spell damage or discover you got value. Your playing around what he becomes the dream isnt to get the best possible minion, but to get something useful. Any cards you get that trade 2 for 1 are value. You will always play him as something useful. I intend to make a gimmick joke rank 25 for fun deck that plays him with shadowstep and gang up with the dream of having a hand of ever changing cards with no desire to win just to joke and fuck around. Why? Because hearthstone is a game and fucking around is more fun than legend. Been there done that legend is boring. 

Mind vision would copy shifter, but duplicate, gangup, sap...etc...would copy the minion he transforms  into. As long as he is in hand he counts as shifter, once he enters play the transform is final. 

The one one copy from shadow caster would transform however it will stay a 1/1 for 1. He would change art and abilities but would not become an 8/8 rag for 1, however, he would be a 1/1 rag for 1. Which isn't bad.

This would be the best time to get him as he keeps the mana reduction. It was confirmed on the battle.net forums. Now don't get me wrong I'm saying getting him on turn two from portal would be  good for him. However, it would be better than a wisp but worse than a 2 drop. 

Something tells me that there will be a brawl full of Shifter Zerus's in your deck