You only get the guaranteed Primal drop:
- With a Seasonal Character.
- Upon successfully completing a Greater Rift Level 70 or higher Solo.
- “Successfully completing” means killing the Rift Guardian before the 15 minutes expire.
- You only get one guaranteed Primal item per Region per Season. Not per Character. Although, once unlocked, they’re unlocked for all your Seasonal Characters. At the end of the Season it will carry over to non-Season.
You can easily check to see if you’ve unlocked Primal drops for the Season by looking at your Achievements. (See image below).
If you have The Primal Soul-o Achievement and a 1200+ Blood Shards cap, then you have unlocked Primal drops.
The guaranteed Primal item that drops is subject to D3’s Smart Loot. There is an 85% chance it will be an item your Class can use and 15% chance it will be for another Class.