Whenever you're dealt damage, you may create that many 1/1 white Soldier creature tokens.
Darien turns your own life total into an army: each point of damage you take makes a 1/1 Soldier. Take damage from pingers, fetchlands, painful mana, and combat to flood the board, then convert that army into wins via anthems, sacrifice loops, or a big alpha strike. You play a grindy go-wide soldier deck that thrives on being attacked and on self-inflicted damage.
Generates a massive token army for free off any damage source, including chump-blocking and being attacked
Punishes opponents who attack you and discourages aggression in general
Mono-white gives access to powerful anthems, board wipes you can rebuild from, and protection
Synergizes beautifully with self-damage engines for repeatable, on-demand token production
Mono-white struggles with card advantage and ramp compared to other colors
Tokens die to the same board wipes white relies on, and you have few ways to protect a wide board
Heavily commander-dependent; if Darien isn't out, your damage does nothing
Slow to get going and weak to fast combo decks that ignore the board