Here is the full list of all 35 QUAKE achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Find the secret message in Tur Torment.
Complete E1M1: The Slipgate Complex on Nightmare without firing a shot.
In deathmatch, electrocute an enemy in water without dying.
Complete Dimension of the Machine in single player.
Find the secret exit in Dimension of the Machine.
Complete Dimension of the Past in single player.
Find the secret exit in Dimension of the Past.
Complete Dissolution of Eternity Episode 2: The Corridors of Time in single player.
Complete Dissolution of Eternity Episode 1: Hell's Fortress in single player.
Complete Scourge of Armagon Episode 3: The Rift in single player.
Find the secret exit in Scourge of Armagon Episode 3: The Rift.
Complete Scourge of Armagon Episode 2: Dominion of Darkness in single player.
Find the secret exit in Scourge of Armagon Episode 2: Dominion of Darkness.
Complete Scourge of Armagon Episode 1: Fortress of the Dead in single player.
Find the secret exit in Scourge of Armagon Episode 1: Fortress of the Dead.
Defeat Shub-Niggurath in single player.
Complete Quake Episode 4: The Elder World in single player.
Find the secret exit in Quake Episode 4: The Elder World.
Complete Quake Episode 3: The Netherworld in single player.
Find the secret exit in Quake Episode 3: The Netherworld.
Complete Quake Episode 2: The Realm of Black Magic in single player.
Find the secret exit in Quake Episode 2: The Realm of Black Magic.
Find the Well of Wishes in the Crypt of Decay.
Kill a shambler with an axe.
Kill a shambler before it is able to cast its lightning attack.
Complete Quake Episode 1: Dimension of the Doomed in single player.
Find the secret exit in Quake Episode 1: Dimension of the Doomed.
Kill a monster with another monster's attack.
Find a secret area.
Complete E4M6: The Pain Maze on Nightmare without taking damage.
Complete Quake on Nightmare in single player.
Complete Scourge of Armagon on Nightmare in single player.
Complete Dissolution of Eternity on Nightmare in single player.
Complete Dimension of the Past on Nightmare in single player.
Complete Dimension of the Machine on Nightmare in single player.