This is for the Company of Exiles event, which makes all of your spells summon a random unit of that cost. To that end every card the opponent will play is a target for Backlash and Spell Swipe, so I wanted to run a full playset of both of them. Plague also seems very good as a way to counter Token strategies and aggressive decks and won't hit your own random unit you get like Lightning Storm will. In addition to our plan to spell swipe and backlash to victory, we get one of the best late game spells in Channel the Tempest. The card is already killing a unit or burning face and drawing 3 cards, but now it comes with a random 8-drop.