I got the same dialogue like you.
And no potion.
So, perhaps this potion existed 2013 at beginn of the quest conception, but was never introduced to the game...
I think, we need a mod...
Level:82, XP:9933730, PV:497, FQ:62
HP:194, AC:288, AD:50-64, AP:3, ECC:34%, CM:4,5, BC:184, DR:2
Gold: 1046288 | RoLS:2, RoL:2, ElyR:1, HoF:1, ChaR:2, GoLF:1, ShaF:2, GoW:1, SRoV:1, VSH:2, WMC:1, BD: 1