Guiltfeeder

Fear (This creature can't be blocked except by artifact creatures and/or black creatures.)
Whenever Guiltfeeder attacks and isn't blocked, defending player loses 1 life for each card in their graveyard.

Commander Legends: Battle for Baldur's Gate

Commander 2016

Fear (This creature can't be blocked except by artifact creatures and/or black creatures.)
Whenever Guiltfeeder attacks and isn't blocked, defending player loses 1 life for each card in his or her graveyard.

World Championship Decks 2003

Judgment

Guiltfeeder can't be blocked except by artifact creatures and/or black creatures.
Whenever Guiltfeeder attacks and isn't blocked, defending player loses 1 life for each card in his or her graveyard.

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.