You can obtain unbreakable captains by branding them while they are receiving an animated attack from an ally orc. The point made in this video is mainly valid for the early game, right now the best way (although not consistent) to obtain good orcs is to brand them during online sieges. It's even possible to get unique orcs again as well as DLC ones without the DLC. This way is more realistic to obtain legends such as no second chance with health regeneration