Stormonu
NeoGrognard
Some Slayers will certainly go for something like that (and in D&D terms, this could be represented by feats or paragon paths), but I want to keep that optional. I want most of the classical adventuring careers and origin stories to work with this framework, and the farmer's lad who picks up an old sword to avenge his village after it was destroyed in a monster attack should be just as justified in calling himself a Slayer as someone who agreed to a lengthy magical ritual to establish his status.
As was I; it could be something as simple as the farmer stating "I swear this day before Pelor, I will avenge my village", and said Pelor agreeing with him, granting him the "Mark of the Slayer" - whether he turn out to be fighter, paladin, cleric, rogue, assassin or whatnot. The Mark essentially marks him as something more than the commoner that he once was so that all who see him would know he was a Slayer. Its the story mark that says "he's no longer a background expendable but a Main Character whose story we will follow"