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