Mânes funestes  

Voleur de soupirs  

Panthère d'Urborg  

1 Esprit de la Nuit

Breathstealer / Feral Shadow / Urborg Panther / Spirit of the Night (Mirage)

This combo came about because the Mirage designers (Bill Rose, Joel Mick, Charlie Catino, Howard Kahlenberg, Don Felice, and Elliott Segal) were interested in trying to find alternate costs for creatures. Wouldn't it be cool if there was a big creature that could be played without paying mana? After examining several different ways of doing this, the team settled upon sacrificing creatures. And thus was born the Spirit of the Nightstalker trio. (The name was shortened because Spirit of the Nightstalker didn't fit on the card.)

I wanted to take a moment to point out one of the subtler elements of these cards' design:
Name, Mana Cost, Power/Toughness
Breathstealer, , 2/2
Feral Shadow, , 2/1
Urborg Panther, , 2/2
Spirit of the Night, , 6/5

The flavor of the cards is that the three creatures form together to make the Spirit of the Night. To reflect this, the designers made the Spirit of the Night's mana cost, power and toughness the combined total of the three cards you sacrifice to summon it.

The popularity (among the casual set) of these four cards demonstrated to R&D that there is a subsection of player that enjoys the occasional combo set.