With its discovery, The Grandfather will join the ranks of Andariel's Visage and Harlequin Crest as the confirmed elusive Uniques to exist in the world. The other three extremely low-chance Unique drops, Doombringer, Ring of Starless Skies, and Melted Heart of Selig POE currency trade , have not dropped.

Grandfather dropped byu/Icy-Heat7531 indiablo4

The Grandfather that dropped came as a reward from a Helltide Mystery Chest. It was a Necromancer on the Asia server that received the rare sword. While even these weapons are subject to randomized stats, it fortunately rolled pretty good for them.

I plopped the screenshot of the item into Google Lens so we could get a better look at its rolls:

We know a bit more about how the rare Uniques drop 
Over the weekend, Diablo 4 Lead Class Designer Adam Jackson provided some details on the extremely rare Uniques. We've listed these below:

They can drop from level 85+ enemies. You can get them anywhere you can get a regular Unique, and they always drop at 820 ipower. We currently have 6 of them in the game. They're realllly rare. 
It's important to keep in mind just how rare these items are. People have been playing Diablo 4 every day for a month now. While most are not high enough level to even trigger the drop, those that are have been farming a lot. Adam also clarified in a follow-up tweet that any Unique drop has a chance to appear as one of these six variants. Therefore, the best way to farm these would be to do content that drops the most Uniques per hour.

The odds for these items remind me of Path of Exile's Mirror of Kalandra. That item is so rare that players may not see it drop for cheap POE currency  thousands of hours. Given that players have amassed 10,000 years of playtime in Diablo 4 and we've yet to find every one of these items, the drop rate sure seems astronomically low.