Flavor: Welcome to the Dark Side. We have cookies.
Commentary: it's a beloved son of Mind Control and all Forbidden cards, a Mind Control that actually scales and is useful at any stage of the game. It's not strictly better, however. You have limited control over it and you can't take an Old God or any other 10-cost minion with it. This seems totally appropriate, as the card pictures a person studying some forbidden things (listening to some corrupted cleric) and converting to the Evil Side after that. More powerful minions (that cost more) are more resistant, but even they'll eventually succumb to these dark gifts. Only the Old Gods and other creatures of mythical power are totally immune.
What I hoped to achieve was a sortoff BGH-ish theme, some sort of counter if the opponents old god goes out of control. How would like to face off against a mega buffed C'thun? But I did not want the instant-kill mechanic that BGH have because that would make this into a higher cost copy.
I put the rest of my thought through the making into a spoiler clamp to shorten down my post a bit :)
I made him an 8/8 for 8 because I want him to be a able body even without the "old god bane" buff. The reason to why the buff is not a battlecry is because if the opponent goes too 12 or over at some point during Neltharions boardtime I want him to react to it. I want this card to be a counter to the old gods. A card that you need to take into consideration when you play a old god deck.
The reason too why I picked Neltharion is because if someone whould be angry at the old gods it should possibly be him seeing as all he wanted was some time to nap but the old gods just HAD to whisper to him for the entire time. That's on the "Put-his-hand-in-a-bowl-of-water" level. Just a super douchey move.
And as some speech options I was considering something like and this is in the human voice of Neltharion: without the buff " The old god filths must die!" With the buff "Graaaa! Face me false god!". Attacking C'thun " You did not see this coming?!" Attacking N'zoth "Corrupt this!" Attacking Yogg-Saron "I will end YOUR hope!" Attacking Y'shaarj "Face TRUE rage!" and on death "Others will come for you.... I.... was only.... the first...." Attacking anything BUT an old god " You are in my way "
This is however my first card so I'd appreciate feedback very much :D
Spell Damage is the least used ability in Hearthstone, but I definitely want Spell Damage to appear more often. Thalnos, Cult Sorcerer, Azure Drake, even Jungle Moonkin and Malygos - there are a lot of cards that could empower Hastur's Battlecry.