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