He's an imperial guard commander!
Al'rahem is a name shared by two Captains in the Tallarn Desert Raiders' 3rd Regiment. One captain led the 3rd Regiment in the Macharian Crusade while the other one leads the current 3rd Regiment. The relationship between the two is unclear; either they're related by family, a result of warp fuckery, or one of GW's endless fuck-ups. Or Al-rahem is in reality a title. PLOT TWIST. Seriously, fuck-ups theory.