This is a PvP lab where the players start with a pre-made deck of random cards and 4 mana. At the beginning of each round, instead of gaining mana, the cost of all player's hand and deck is reduced by 1, down to a minimum of 1. This lab was made available in patch 2.2.
"It's chaos in the Lab! The Mana Generator's offline and the Cost Reducer's gone haywire. Choose the right cards to your increasingly-discounted deck and defeat your opponent in the name of science!"
Each player starts with 4 max mana, and a small premade deck with a few random champions, spells, and followers. At Round Start, instead of gaining an additional max mana, the cost of all cards in each player’s hand and deck is reduced by 1, down to a minimum of 1. Each Round, the attacking player receives a 0 Duplicator that lets them choose one of 4 random cards to add to their hand, with 2 additional copies added to their deck.