But I thought it was just "overgeared mythic raiders" according to mmoc lfr heroes:
Argus the Unmaker
Cheat Death and similar effects will not reset their cooldowns when you die during the final phase of the Argus the Unmaker encounter.
Boss Health increased by 10% on Normal and Heroic difficulties
Soulbomb and Soulburst now deal periodic damage more rapidly
Edge of Obliteration damage increased by 50%
Constellar elemental vulnerabilities reduced from +200% to +100% damage taken