Sources of Ancestral gear.
Ancestral gear is the pinnacle of Diablo IV items now. If you're struggling to get some, here's a couple of reliable ways.
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Diablo IV: Vessel of Hatred changed up the itemization a good amount. Say goodbye to Sacred items (no one remembers those anymore). The pinnacle of power comes from Ancestral items, so once you get to the endgame, you will start seeing those.
Here's a couple of quick and easy facts about Ancestral Items in the Diablo IV expansion to know.
They only start dropping in Torment difficulty
They have 800 item power (highest in the game) so you know there's nothing higher
They will always have at least 1 Greater Affix and can have multiple
They can be Tempered up to 12 times
Both Unique and Legendaries can drop like this, and they don't drop very often at all. It will take some time to even find your first Ancestral item.
Also, every Mythic Unique item ("ubers") will drop with a Greater Affix as well. When you finally get one.
There are many methods to finding Ancestral items; as typical for Diablo, they can drop from most activities or monsters in the game. However, I was pretty stuck on Torment I at first because I couldn't find an Ancestral weapon. I was trying Helltides with the Accursed Ritual boss, Blood Maiden, thinking that would be a good method, but they wouldn't drop. Barely any items would drop that are Ancestral.
However, there is a surefire way to get one: Infernal Hordes are a guaranteed Ancestral item drop in Diablo IV. This activity is fantastic anyway, as you can still get materials or gold if you desire with remaining Aether, and you will get tons of XP, which will benefit your Paragon Board progress early on. Just make sure to farm up some Glyph XP from Pits earlier to have them active for easier demon slaying.
Other activities have varying chances of rewarding you with Ancestral items. Note that higher Torment levels don't increase the chance of getting them, but they simply award more loot. So that's more chances to get Ancestral Legendaries or Uniques.
Here are some other ways:
Helltide Chests, Purveyor of Curiosities, Nightmare Dungeons: Rare
Whisper Caches: Uncommon
Kurast Undercity: Rare
Mercenary Caches: Uncommon
Dark Citadel: Common
In the end, there's no surefire way other than Infernal Hordes currently. However, Blizzard is still tuning the game, and they have been buffing Kurast Undercity in recent changes, for example. So make sure to check patch notes to see if any activities have their rewards updated.
Ancestral gear is what you need to take on the endgame, so get in there and start grinding. Make sure to do so equipped with a wireless SteelSeries gaming headset like Arctis Nova 7 for optimal sound.