Marisa, Dark Cosmic Seller

At the start of your first combat step, add {b}{b}. Spend this mana only to cast spells during the combat step.

When you spend two or more mana during a combat phase, you may place an "Unstable Potion" token into your deck, then shuffle.

Unstable Potion is a Token with: When you draw this card, reveal it. Deal 5 damage to a random target, then draw another card.