found throughout the 
Memory of Flash Concentration is a shiny new legendary power available to us in Shadowlands. Obtained from Grand Proctor Beryllia in Sanguine Depths, this legendary power can be crafted onto your Neck or Wrist. Equipping it causes casts of Flash Heal to grant you a short 20-second buff stacking up to 5 times that both reduces the cast time of Heal, and increases its healing. Flash Concentration changes how you prioritise casting your filler spells, placing an emphasis on Heal. When played well it can provide an immense amount of efficiency and HPS. When played poorly it can lead to lower HPS and mana starvation.
Flash Concentration really turns Heal into a filler powerhouse. Heal becomes stronger than all the other filler options in terms of both HPS (throughput) and HPM (efficiency).