Divinity: Original Sin - Enhanced Edition

You'll find Blossious outside of Cyseal on the road to the church and just south of the lava section. He's in a little worn down shack. Speak with him and go through his dialogue options. He will hand you his will and ask you deliver it to his wife (widow?)

Once you reach Luculla Forest, you can head to Silverglen, which is a short walk to the north. Head to the local tavern and you'll find Marisa walking around the ground floor. Once it's been delivered unaltered, you will get the achievement.

When I completed the achievement, I didn't even read the will. So I can't tell you if you can read it, leave it unaltered and still get the achievement. Might be worth just leaving it to be on the safe side. If anyone wants to clear it up in the comments, feel free.