Creature cards that were put into your graveyard from any zone other than the battlefield, such as a card that was discarded, won’t return to your hand.

Creatures dealt lethal damage at the same time Fell Shepherd deals combat damage to a player will also be returned to your hand from your graveyard. If Fell Shepherd is such a creature (perhaps because it gained trample, attacked a player, and was blocked), it will be one of the creature cards returned to your hand.

You can choose a creature as the target of the last ability and sacrifice that same creature to activate the ability.