#1. Psychic Scream

Expansion: Kobolds & Catacombs
Psychic Scream is probably the best board clear Priest has received in Arena. It is one of the best cards in Arena. Cards like Violet Wurm are very common in Arena and if you psychic scream those 1/1 guys, you will restrict your opponents' draw by a lot.
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Draw mechanics and deck thinning are barely present in Arena, so if you shuffle a lot of minions into your opponent's deck, especially weak token ones, your opponent has a hard time drawing the right cards.
Honourable Mentions
Curious Glimmerroot

Expansion: Journey to Un'Goro
Curious Glimmerroot has an interesting "guess" mechanic and its power level is similar to Crystalline Oracles. The card choices are always Class Cards. You might be able to guess it if you saw your opponent play that card or if you played enough to know what's common. It has a fairly statted body to go with it too.
Shadow Visions

Expansion: Journey to Un'Goro
Shadow Visions is just as good in Arena as it is in Constructed. You can discover copies of Mind Control, Holy Fire, Free From Amber and other extremely high power level spells if you drafted them. Priest has a lot of useful removal mechanics and it is highly likely to draft a good spell. Shadow Visions will be a hard-hitting loss for Priest.
Devour Mind

Expansion: Knights of the Frozen Throne
Devour Mind adds 3 different cards for the cost of 5 mana. In Arena, discovering copies of synergizing class cards or even neutral minions is very helpful. If you run out of cards in your hand, you can recover 3 more cards with this spell. It has the element of RNG to it, but it is a pretty good draft.
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