The Dragon clan Capture mechanic

[Ulminbore]

Has anyone any experience of the Capture mechanic used by the dragon clan? There's an achievement, "You Will Fight for me" which says capture either a Myrkalfar, Jotnar or Kobold, while the Capture tool-tip doesn't mention Kobolds. Regardless of which, I've killed (and injured) uncounted numbers of all three, whilst in possession of the lore, and no sign of capturing one (no pop-up congratulation, no increase in Thralls, no Achievement). I know it only says a "chance" of capture, how long are the odds??! Or is there something particular I'm supposed to do?

[Ulminbore]

Okay, after repeated battering of the Myrkalfar camp (but never enough to destroy it) the Achievement has now popped. No idea why. No increase in Thralls, no message to say I'd made a capture, no Myrkalfar in my war band. Anyone got any ideas?

[Locklave]

I read this is a 10% chance per hit on enemies below 33% health that are valid slave targets. Very low chance at very low hp.