First, the Trueheart Duelist is a 2/2 creature that takes 1 colorless and a White mana (1W) to cast. The Duelist can block an additional creature each combat. This is relevant because the Duelist can potentially trade with two creatures at once in combat. But, tthe Embalm mechanic is where the beauty of this card comes in.
On his own, Trueheart Duelist looks like a really good creature, especially in draft. Being a fairly efficient two-mana creature with the Embalm ability, Trueheart Duelist even found itself in a few budget Standard decks. too.