Billy was a human raised in northshire Abbey, receiving a faithfull education, learning the power of Light and becoming a priest who will spread the faith and help the people in needs.
He grew up and went in missions all around the world, healing people in Eastern Kingdoms, Kalimdor, Outland, Northrend, Pandaria, and Draenor.
But when the Legion came in, it was too much. He saw so much deaths, faced so much horrors, that he lost his faith. He turned a bit dark, and started using his power to attack and damage instead of helping and healing. No longer he believed in Light, but started to praise the Shadow.
He is now going in front of ennemies more and more powerful. Becoming confident in his talent with Shadow, and in need of better power, he tried to go beyond.
So, he started using the power of the Void.
But Void is an unnatural thing for a human being. Their energy is unstable, that's why Billy can't always use it, he needs to recover is sanity from time to time.
But there's no concession when you face the Legion. No sacrifice is too big. He needs to control the Void's energy longer, so he can be more useful for his friends.
So he makes a pact with the Old-Gods.
They will unleash his true potential, make him understand better how Void's energy works, so he'll become more powerful than any human being before. But he knows it's not a two-way trip. At the moment he will accept this deal, he will be infused with powers that a human body can't handle. So he will have a moment of pure power, and it'll require all of his concentration to use that power and not let it consume his body.
But will come a moment when all of his efforts won't be enough. And he'll die. But he is ready for this sacrifice, and he will delay this moment as long as he can. That's what facing the Legion means for him.
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This is what I see every time I look at a Shadow Priest using Surrender To Madness.
For me, it's the best class-specific design ever made in WoW. It's so deep, it's one of the only spec where you feel the background story while playing.
I'm not saying it has the perfect gameplay, I'm not saying it's easy to play, or too hard, or whatever. I'm just saying that having a gameplay that have implications lore-wise is insane !
Even that "reaching 3rd Void Torrent" goal that you have in each fight is amazing. It's like your character feels that he is going mad and he will die soon, but for the sake of his friends he is ready to put his madness further and further and give everything to do that extra damage that'll help them knock the boss down.
I know that some people think it's too hard to play, or not fun and I understand them. But... Man, this gameplay design is genius.
So, what I wanted to say : please don't change the Shadow Priest and S2M gameplay too much. Don't change the insanity generation or this kind of things. Don't try to make S2M a niche choice. If needed, put some caps on Mass Hysteria or whatever you want to lower the best players' damage, but Blizzard, this gameplay is the most meaningfull gameplay you ever made, so please don't destroy it.
Let's make the Old-Gods wait again before Billy finally give them his soul.
Thank you.