It sets your HP and MP to 9999 Max, I used it about 2 hours ago. Gave me 300 hp/mp took a while but I managed while soloing the oboron from Mis coast. Tier III nukes on rdm were 3500 - 8k dmg each nuke. Solo'd the NM in about 30 seconds
You can get Primeval Brew as a temporary item in Walk of Echoes, but it's completely random and very rare.
It's even more awesome than in Abyssea because WoE NMs don't have damage-absorbing TP moves, so you don't have to hold back. It's a little less fun when you get hit with amnesia, though.