Mass Hysteria

Mass Hysteria is a rare priest spell card, from the Rastakhan's Rumble set.

Strategy
Mass Hysteria is often best used when there are no minions on the priest's side of the board, making it a decent board clear. It can also be used after having your minions attack the enemy hero, allowing your minions to attack twice in a turn.

It is hard to tell the result of Mass Hysteria when there are many minions with different stats, especially how to raise the probability to clear the board. But below are a few easy ones.
 * When there are 2 minions, they would collide once or twice, depending on their Healths. For example, Mass Hysteria would perfectly destroy an 8/8 and a 4/9.
 * When there are more minions, for example:
 * 1/1, 8/8, 8/8: 8/8s are destroyed for sure, 1/1 has a (1/3) chance to survive.
 * 4/4, 4/4, 8/8: 4/4s are destroyed for sure, 8/8 has a (1/3) chance to survive.
 * 4/4, 5/5, 8/8: 4/4 and 5/5 are destroyed for sure, 8/8 has a (1/3) chance to survive with 3 health.
 * 8/8, 0/2, 0/2, 8/8: probability of survivals of 0/2s are complex, but it is easy that 8/8s have a 1/12 chance to survive.
 * If any collision would cause both minions die: Twisting Nether if the number of minions is even, Brawl if it's odd.

Mass Hysteria often works as an excellent board clear against Aggro decks.

Priests can play Deathrattle minions like Cairne Bloodhoof, Coffin Crasher or Violet Wurm before the turn playing Mass Hysteria, which can almost destroy all minions while maintaining deathrattles. Also, Deathrattle minions can be played to counter this.

As friendly Overkills can trigger during Mass Hysteria, this combos well with Linecracker played last turn, being able to get an extremely high attack.

Lore
Mass Hysteria shares its name, though none of its functionality, with, an artifact trait available to Shadow priests wielding the artifact weapon during . Mass Hysteria caused the priest to gain a stacking buff that increased the damage dealt by their Shadow Word: Pain and spells as long as they remained in Voidform. As with all artifact traits, Mass Hysteria was removed when artifact weapons became deprecated in .

Trivia

 * According to Peter Whalen, Mass Hysteria was originally conceived for an earlier expansion, possibly The Witchwood or Knights of the Frozen Throne, and had the effect "Each minion randomly attacks another character", meaning they could hit heroes as well as minions. The designers "played that for about two games and said 'We hate this card'" and removed it from the set it was planned to appear in. During the development of Rastakhan's Rumble, they returned to the card and changed it to only hit minions, which made far more sense and ended up being a lot more fun to use. They had no idea how good the spell would be or what the appropriate mana cost would be, but eventually landed on 5 mana as the right spot.
 * According to Dean Ayala, most of the time the team tries to avoid creating cards like Mass Hysteria which can be difficult to understand at first glance. He explains: "even very simple boards that have three minions, you kind of look at the board, then you look at the card, and you're like 'Wait, what the hell’s going to happen here? Is it going to clear everything?' [...] And then you just cast it. Most of the time it's like 'oh, that was something good kind of happened there.' [...] I think because it mostly does what you think it's gonna do, it might not be the exact outcome that you thought it was going to be. But it didn't turn out to be so confusing that you really had no idea, conceptually at all, what it was going to do. It was just sometimes it wasn't the outcome you were thinking of, but it turned out to be okay."

Patch changes

 * Added.