My first run was with Shadowheart as PC and I went down the Dark Justiciars path all the way (which was a blast and pretty dark).
A proper build isn't easy. I went for
1 Rogue / 4 Cleric / 5 Ranger(Gloom) / 2 Rogue(Thief) to make her a sneaky dual wielder.
first level of Rogue gives proficiency with shortswords and expertise in stealth. 5 Gloom gives extra attack and Dread Ambusher. 2 Thief gives extra bonus action for dual wield. Several very good shortswords can be found along the way.
On the topic of armor it's worth to point out that you get a complete Dark Justiciar armor set if you follow the dark road of Shar. A very rare Helmet (Covert Critical) and Half Plate (AC 17, Advantage on Stealth and Constition checks, extra Necro damage) and rare Gauntlets (Beckoning Darkness spell, extra Necro damage) and Boots (Shadow Teleportation). Besides the extra features the armor has the iconic Dark Justiciar look of course
that can help you convince Dark Justiciars to side with you when overthrowing the Mother Superior in Act 3.
Also you can get the legendary Shar's Spear of Evening (Blind immunity and create darkness while attacking) and later in Act 3 the legendary shield Viconia's Walking Fortress (AC +3) when following Shadowheart's quest line.
These items made me try a strength based build (6 Cleric/6 Fighter), but ultimately respecced back again, because I just liked the two-weapon fighting better (and of course there's something irresistible about Orin's blades haha).
So yeah, Cleric of Shar / Shadow Monk sounds fun, very thematic and fitting for Shadowheart, just that you'll give up on the Dark Justiciar armor set.
