Bosses
Mephisto
Exalted boss at the top of Atanos in Skovos. Torment X or higher.
Where and how
His lair is the Echo of the False Prophet, at the very top of the Atanos area in the Skovos Isles. You need Torment X or higher. As with other lair bosses, the fight itself is free: the Crux of the False Prophet, his own mythic key, opens his hoard. The Crux is rare. It can come from greater lair bosses at high Torment and from War Plans caches, but drop rates are not published.