Dack is a pretty good card when he has no synergy at all, he is great when he is fueling delve and such like
I have seen it played in Naya and Jund type decks but I am not a huge fan of that, if you can't cast it on turn three or when you have three mana the gang starts to look a lot worse
As you might expect, having 90% of Standard players running Jace, the Mind Sculptor, 70% running Stoneforge, and perhaps 50% running both in Caw Blade did some crazy things to card prices
As a result, the card has been banned in Modern, Legacy, Pauper, and is restricted in the Vintage format
This is extra strong if you have one Witness on the battlefield with another spell recursion creature
When this Aura is put into a graveyard from the battlefield, return it to its owner's hand