Complex sentence: However, Maxtible, who has travelled to Skaro through the mirror cabinet, is discovering just how ruthless the Daleks are and how empty their promises can be; he is tortured for his failure to bring the Doctor to them.
Split into simpler sentences: However, Maxtible is discovering just how ruthless the Daleks are and how empty their promises can be. Maxtible has done the same earlier, and is tortured by the Daleks for his failure to bring the Doctor to them.