Lion's Eye Diamond trades your entire hand and the artifact itself for three mana of one color. That is a severe cost for an ordinary deck, but strategies that cast from the graveyard, use cards in exile, or operate through a spell already on the stack can turn the exchange into a powerful burst of mana. It belongs in a deliberately constructed engine rather than serving as general-purpose acceleration.
Underworld Breach is a notable partner because cards discarded to Diamond may remain available to cast through escape. In a blue-red deck, Brain Freeze can help supply additional graveyard cards while contributing to a larger spell sequence. The exact line depends on your starting resources, available cards, and escape payments, so the Diamond should be evaluated as one part of the engine.
Its activation restriction is crucial. You may activate it only when you could cast an instant, which means you cannot use it in the middle of casting a spell to pay for that spell from your hand. You can respond to a spell or ability already on the stack, discard your hand, and have mana ready when that spell or ability resolves.
Plan the sequence before sacrificing it. The mana must have a useful destination after the discard, and losing your hand leaves little room to improvise if the key effect is disrupted. Diamond is strongest when your deck intentionally turns discarded cards or previously established casting permissions into usable resources.
