We have some major changes coming to Mythic+ in patch 11.1 and Season 2, as the old affixes have been redesigned and we get to see what the new one is!
Blizzard shared the changes in today’s 11.1 development notes, 
Xal’atath’s Bargain: Voidbound got completely changed, as 
Dark Prayer now has a long cast and grants the new Xal’atath’s Gift buff on cast completion. Meanwhile, 
Xal’atath’s Guile got buffed to give enemies 2% more damage per keystone level.
- Unified enemy buffs on all Xal’atath’s Bargain affixes with Xal’atath’s Gift, increasing damage done up to 50% and reducing damage taken up to 100% for 20 seconds on all nearby enemies in combat.
- New Affix: Xal’atath’s Bargain: Pulsar – While in combat, Xal’atath summons pulsars that orbit players. Replaces Xal’atath’s Bargain: Oblivion in Season 2.
- Xal’atath’s Guile has been redesigned – Now causes keystones above level 12 to increase health and damage of enemies by 12% per level instead of 10% per level.
- Xal’atath’s Bargain: Voidbound has been redesigned – Dark Prayer now has a 15 second cast time, and grants all nearby enemies in combat Xal’atath’s Gift when the cast completes. No longer grants damage reduction while active.
- Potency of Xal’atath’s Gift granted by Dark Prayer is based on the remaining shield percentage on the Void Emissary.