Shows on a hostile target with <= 50,000 HP and when you have less than 28 soul shards in your inventory
Bonus macro that will discard the last soul shard in your soul bag to avoid shards spilling out of your soul shard bag (bag must be in last bag slot):
/cast Drain Soul(Rank 1)
/run local a=GetBagName(4); if a=="Ebon Shadowbag" or a=="Felcloth Bag" or a=="Soul Pouch" or a=="Box of Souls" or a=="Small Soul Pouch" then PickupContainerItem(4,GetContainerNumSlots(4)) DeleteCursorItem() else end