Here is the full list of all 63 Divinity: Original Sin (Classic) achievements.
You sent the Watch on a wild goose chase.
You have killed Arhu to get hold of the soulforge repair ritual.
You have made it into the Source Temple.
You have created a spell that makes Death Knights vulnerable.
You have saved the Homestead at the Shelter Plane at the End of Time.
You have destroyed Balberith.
You have solved Bellegar's maze.
You told Arhu about Thelyron's machinations.
You have safely entered Phantom Forest.
You rescued the prisoners and helped them escape.
You brewed a fine whisky for Hershel.
You made it into Hunter's Edge.
You talked to Icara and learned more about Leandra's plans.
You have rescued the White Witch.
And water. And air. And earth.
Get Brandon to tell you the truth about the mining guild.
You kept the Tenebrium for yourself.
You safely returned to the surface.
You came face-to-face with Mangoth himself. MANGOTH!
You have united the well brothers.
You escorted the whole party safely to Silverglen.
You have met the Conduit.
You delivered Blossius' will to Marisa totally unaltered.
You talked Ebenezer into... fighting you.
Sam and Maxine are going to be wed!
You passed the test! Well, one of many.
You advised Desdemona to forgive Samson.
You killed the ghost of Pontius Pirate.
You pushed this one button we wanted you to push. Congratulations!
You found a job for the unemployed. They will work for Captain Jack.
You found a way to open the barrier without harming any mushrooms.
You saved Bonnie!
You informed Aureus about the source of the undead.
You defeated the four elemental demons, unlocking the prison of...
You kept Cecil's staff for yourself.
You defeated Evelyn and found Councillor Jake.
You have solved the puzzle that we call the Mirror Dungeon puzzle.
You lied to Aureus and said that his soldiers are true heroes.
You met a talking shell called Ishmashell.
You've found the legendary teleportation devices of Rivellon!
You've found your primary suspect.
A certain potion wore off and that had some consequences...
You tried to sneak past the guards... And failed!
You found a job for the unemployed with the Fabulous Five!
You returned the mayor's mighty staff.
You delivered the will to Marisa... After changing it in your favour.
Esmeralda was freed from prison.
You destroyed Snorri's ball outside of the time limit.
You made Archibald king of the trolls.
You made it so that Loic couldn't escape and had to fight you alone.
You have killed Cassandra to get hold of the soulforge repair ritual.
You have found Zandalor.
You have made a deal with the demon Balberith.
You have offered one of your own to the demon Balberith.
You sent the Watch after those annoying imps.
You have finished the Trial of Ascension.
You have met the legendary weresheep!
You have totally, completely, and utterly finished the game! Woooo!!!!1
You told Aureus that his soldiers are cowards.
The Fabulous Five packed their bags and left town.
You advised Desdemona not to forgive Samson.
You talked Ebenezer into... Not going to the Hall of Echoes.
You thwarted Billeh Gahr's trap with teamwork.