The Gloomstalker Ranger and the Assassin Rogue are two great damage-dealing subclasses that are extremely useful in Baldur's Gate 3; high initiative makes the package even more appealing. I like classes that have a lot of versatility, so the Gloomstalker Assassin has become one of my favorite additions to a party in Baldur's Gate 3.
BALDUR'S GATE 3 Gloomstalker Assassin Multiclass Build Guide (with Magic Items!)
The Gloomstalker Assassin build can be great for your main character, but it can also be a great choice for one of your companions, with Astarion being a natural choice. Here's how to create the Gloomstalker Assassin build.
To make your Gloomstalker Assassin as effective as possible, you'll want to focus on Dexterity to hit more with finesse weapons, have higher Initiative, and increase your AC. You'll also want some Constitution for your health and Concentration saves. Finally, you'll want your Wisdom to be somewhat high to favor saves and Ranger spells.
To start, take Rogue for your first two levels so that you have Sneak Attack and Cunning Actions, which are very useful early on. After that, take Level 3 Rogue to unlock the Assassin subclass, or start dipping into Ranger. In general, you want to get to Rogue Level 7 and Ranger Level 5.