Malah's Potion is a quest item used during the Prison of Ice quest during Act V of Diablo II.
After finding Anya encased in ice by Nihlathak, Malah will give you her potion to thaw Anya.
It is not specified whether Malah's Potion is a more-potent variant of a Thawing Potion, or a counter-spell to the curse used to freeze Anya.
Diablo I
Recovery — Healing Potions • Mana Potions
Elixirs — Elixir of Strength • Elixir of Vitality • Elixir of Magic • Elixir of Dexterity
Quest Potions — Golden Elixir • Spectral Elixir
Demo only elixirs — Elixir of Clumsiness • Elixir of Disillusion • Elixir of Weakness
Diablo II
Recovery — Healing Potions • Mana Potions • Rejuvenation Potions • Antidote Potion • Thawing Potion • Stamina Potion
Throwing — Poison Potions • Fire Potions
Quest Potions — Potion of Life • Malah's Potion
Diablo III
Recovery — Health Potions