The Grandfather is one of the 6 rarest Unique items in Diablo 4. It's one of the strongest and most sought after items in the entire game.
How to Obtain - Random Drop Monster lvl 85+
Rarity - Uber Rare
As mentioned above this item is exceptionally rare and is impossible to target farm or obtain in any way other than luck. It can only drop from monsters that are level 85+ meaning you're going to need to be in late endgame content before it can even start dropping. The drop rate of this and it's fellow Uber Rare Uniques may be adjusted in these early days of Diablo 4 but for now don't get your hopes up.
The Grandfather
The Grandfather Mechanics
Many Unique items change the way skills and builds work in significant ways. More info coming soon!
For further information, see our Unique Item Wiki