Betrayers of Kamigawa, Rare. From a mechanical standpoint, making any other permanent indestructible is always desirable. But on top of that, That Which Was Taken’s divinity counters have synergy with the Myojin cycle from the Kamigawa block, allowing them to use their active abilities additional times.
Look that combo up… suffice to say, things can get silly if it happens.
Lore-wise, this card is the entire reason behind Kamigawa block’s story! While the block itself left a lot to be desired in the power department, it had flavor in spades (on a separate note, I highly suggest trying to find the Kamigawa block novels, they are really worth the read). That Which Was Taken was the kami child of O-Kagachi, the most powerful spirit on Kamigawa. Its theft by Lord Konda was the event which sparked the war between mortals and spirits.
Seriously? This thing’s daddy was the spirit of the barrier between the mortal world and the spirit world. WHY WOULD YOU THINK STEALING THE CHILD OF A PSYCHOPOMP WAS A GOOD IDEA???