To spell things out:
The inspiration for the SoulForge are the various monster generators from fiction, but most precisely, the ones from the old video game, "Gauntlet."
Fight all you want, kill all you want- until the monster generators are destroyed, they keep generating monsters.
Or, if it helps you- think of the movie "Dragonslayer," in which the first attempt to slay the dragon fails, and just as the village thinks they have been rescued from the tyranny of the drake, it returns, EXTREMELY P.O'ed.
So yes, let them think they've succeeded, until reports of the return of the undead reach them again. You could even run an interim adventure or two before reports of graverobbing and missing people in the area resurface.
If, after a second visit to the SoulForge they don't get it, employ the old "Mystic Seer" who will tell them that "This place must still be cleansed!" Perhaps you should even let them witness the SoulForge spit forth some undead.
Because, lets be honest, nobody's perfect, and sometimes even the best of us can miss clue after clue unless they're made as obvious as a group of polar bears playing Danzig tunes at a formal state dinner in Egypt.
