|Battle Cry||FAREWELL! (not really)|
|Founding||First Founding (later retconned)|
|Primarch||One of the lost/Unknown|
The Valedictors are a Space Marine Chapter from the early days of Warhammer 40,000. They were first introduced as a First Founding Chapter, who fought Orks, Eldar, and Chaos Space Marines (the latter during the Horus Heresy).
When the Second Edition's major fluff revisions were made, the list of First Founding Chapters no longer included them, and they were later mentioned to have come from one of the "Later Foundings." General fanon holds they were Lost Legion Loyalists brainwashed into believing they are from another legion after the lost were purged.
Their name means "those who say goodbye" or "givers of farewell addresses." Presumably, this means they're masters of the action hero one-liner. Perhaps their demotion caused them to get really angry...
Alternately, if you prefer Badass to Fun, you could take it as a poetical way of saying executioners, since "the last goodbye" is a common euphemism for death, particularly in an old-timey honorable context. Once you meet the Valedictors you aren't going to see anyone else ever again.