During the War of the Spark, Dack fought the Dreadhorde with magic that magnetised its lazotep armour. Then an Eternal caught him from behind. He damaged it and tried to planeswalk, but the reactivated Immortal Sun held him on Ravnica.
The creature harvested his spark. Dack died as that stolen power went to Bolas, while the fighting continued around him. His death is an actual loss in the war, not something that can be inferred merely from a missing card or a planeswalker leaving a battlefield.
The Echoverse offers a philanthropic Dack partnered with Sifa Grent. He belongs to another reality, not an escape from the original's fatal harvesting.