I'm really doubting Zaros being Sliske's master because of the events in Fot* and also because of the manner in which Sliske's master speaks. Sliske literally betrayed Zaros in that quest by convincing us to sabotage his return. It doesn't make sense to me that Zaros stills wants to give orders to Sliske when he "has us" after the quest. The other thing related to that unknown voice is that it sounds too rude and impatient for Zaros. Zaros spoke to us patiently and kindly and even to Sliske despite the betrayal.
I think Guthix could be ruled out too since he is dead and he is not nihilistic and angry like the unknown voice. In addition of that, Sliske wants destruction in Gielinor whereas Zaros and Guthix don't want (Zaros wants more targeted due to the elder gods, but I think Sliske wouldn't enjoy that. Although Zaros is Machiavellian, to me he doesn't fit for Sliske's master anymore).
Kerapac has become in my opinion a good candidate after re-reading multiple posts related to him. Both the voice and him are quite rude and impatient (Kerapac can be quite eager to break the curse). The voice could also belong to a Necrosy*te dragonkin because by killing all the false users (many of the gods) the curse eases.
I'm not sure if examining that Sliske's Endgame artwork where the Stone of Jas breaks apart behind smiling Sliske could have an answer; the Stone of Jas might break up (if I recall right, there has been a thread about that theory...) so Sliske's master might want to destroy the Stone, so it fits well that Kerapac is working with Sliske. Also note that we went to Daemonheim in Kindred Spirits and discovered there a dragonkin lair before getting caught by a Sliske's wight. In One of a Kind, we went to visit Kerapac's laboratory behind Daemonheim so it may be so that Kerapac owns that prison... and Kerapac was also arranging Sliske's Saw-like game in Kindred Spirits.
Loristic, the Seeker of Lore, the Archivist of Knowledge, the World Guardian, the Diplomat of Cabbages, and the Regent of Miscellania. May
anima mundi
be with you and hail Guthix!
24-May-2016 19:52:02