Assassin Gloomstalker dual wielding hand crossbows, and taking a swig of Elixir of Bloodlust. That's 8 attacks on turn 1, assuming he's initiating combat from stealth and killing an enemy.
Gloomstalker assassin action surge Durge with club of giant strength and titanstring bow is my solo honor mode build. Nuke an enemy and go invis with the Durge cape, drop out of combat and repeat. You can take out the whole game with this strat. I like running either halfling or deep gnome since they get stealth advantage and good saving throws.
Really, because he's like one of my main damage dealers, especially if I start the combat. That's like 4 attacks in a row, and at least 1 of them is gonna crit, and 2 of them are sneak attacks.
I don't want to spoil anything, but if you decide that his back story has twisted him into a vengeful madman, you can uhh...get pretty crazy powerful with him late game.