Amrita Vein is a rare herb required to upgrade Regeneration Potion, Lyrium Potion, Tears of the Dead and Mighty Offense Tonic.
You may not encounter this herb early on the game because it’s actually located on the Hissing Waste, which is a high level map. If you do unlock The Hissing Waste, the map below will show where to find Amrita Vein. The white dots indicate the Amrita Vein herbs.
There’s a good amount of Amrita Vein just north of the Sand Crags Camp as circled above. The amrita vein on the mid northern part are also pretty safe to venture since the area is vast and there’s really no aggressive enemies, except lurkers (if they do spawn). There’s also amrita vein down the valley on merchant area (spiders might be lurking by there).
The good news is that you will only need around 10 Amrita Vein to upgrade all the said potions. To see what upgrades require amrita vein, see the list of potions, tonics and grenades.