At the beginning of your upkeep, you may discard a card. If you don't, sacrifice Lim-Dûl's Paladin and draw a card.
Whenever Lim-Dûl's Paladin becomes blocked, it gets +6/+3 until end of turn.
Whenever Lim-Dûl's Paladin attacks and isn't blocked, it assigns no combat damage this turn and defending player loses 4 life.
4/15/2013 An ability that triggers when something “attacks and isn’t blocked” triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.