User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-4696134-20121231155953/@comment-4902593-20130101093008

After the Titan war Kronos was severely weakened and most likely confined to Tartarus. So I don't think he still retains Luke's form. As for the cells you're talking about ... if monsters were confined in these dungeons they would not have captured Nico in the first place. So yeah, definitely no cells. There may be a possibility that Percy and Annabeth would meet Luke in Elysium (after returning from Tartarus) and the meeting won't go so bitter (because it is hinted in the books that Percy is able to sympathize with Luke much better).

P.S: In the Lightning Thief the Nereid mentions to Percy that  Houdini "could even escape the depths of the pits of Tartarus." So ...?