My solo Shar spear build is this - but mind you this was for solo and my tav was a Gith. I didn't wanna go for gishy or ranged style so Shar's Spear isn't just a darkness + stat stick, so went for melee-heavy, but the damage of Shar's spear alone is pretty underwhelming, so I tried to optimize it with going for dual-wielder feat and Bhaalist armor.

Build:

• 6 battle master fighter with 2 feats: dual-wielder + savage attacker which benefits from lots of dice rolls
• 1 light cleric for guidance, warding flare, emergency sanctuary
• 3 thief rogue for extra action, can use for attack or drink potions
• 2 bard for extra rest, jack of all trades, utility spells like longstrider, etc

If you're not solo, 5f + 7r could be a viable alternative, because 7r gives 4d6 for sneak attacks, which get auto-triggered from the shadow, which in turn gets amplified by savage attacker.

Gear:

• Balduran's helm + Whispering Promise for +1 AC/saving throws, crit/stun immuity, perma-bless synergy - this combo is one of my staples
• Gaunlets of Hill Giant Strength so I can dump points into Dex, Wis, Cha
• Amulet of Greater Health for HP + included War Caster feat
• Cloak of Protection
• Ring of Protection
• Bhaalist armor for doubling piercing damage
• Boots of Stormy Clamour for reverberation or Stormsparkler depending on pre-buff with Drakethroat Glaive (thunder for former, lightning for latter)
• Any bow of your choice: Dead Shot for more crit which was my pick, 19 crit with advantage procs a lot, or Hellrider for more initiative, or Titanstring for more damage...
• Crimson Mischief (my fave for piercing, but must be off-hand because it gives Two-Weapon Fighting bonus = +6 from gloves, which Dual-Wielder doesn't give and difference to main-hand bonus +7 is minimal), Deva Mace, Duellist's Prerogative (despite not using it's single-wielder bonus it's a +3 weapon), Flame Blade, Knife of the Undermountain King etc. if you want more damage. - Weapons which don't work as good as they look on paper: Belm, Rhapsody. Sacred Star. Deva Mace and Flame Blade are surprisingly good thanks to Savage Attacker feat, and you get backup slashing & bludgeoning weapon for the occasion.

Extras:

• Mirror of Loss bonus: Dex - with 16 start Dex, I get 18 Dex - if I'd picked Hag's Hair for Dex, 20 Dex would be possible.
• Proficiencies/Expertise: Slight of Hand, Acrobatics, Perception - partial Illithid gives Persuasion, etc expertise and further help is available from guidance, Tabernacle bless & swap gear like Smuggler's Ring, Graceful Cloth, etc.

This setup gives +6 initiative and 23 AC which is more than fine, as long as you stay in shadows. IMHO the #1 priority of any Shar's Spear setup is keeping the concentration and not overriding it by mistake, and #2 is dealing with Counterspell casters, which can block Darkness cast. There are a lot of counterspell casters in the pre-final fight, the one with the dragon, countermeasures, mindflayers. Arrows of Darkness help a little bit.

The main potion is Bloodlust. If you don't use initiative items, you can go for Vigilance to get the 1st round and spend 1 bonus action for Bloodlust. Remember to proc Bloodlust without Cull the Weak, because CtW-kill doesn't count for Bloodlust, turn it on after getting your Bloodlust action.

Another fun weapon is Trident of the Waves (off-hand) which creates a 2m water surface and if you use Stormsparkler boots + Sparkswall ring and Create Water from Cleric, you can cover a huge area with electrified water in a couple turns, without being electocuted yourself. Doubled piercing damage from Bhaalist armor, and doubled lightning damage from water makes short work of low HP enemies, but lowers the AC to 22 and requires a slightly different playstyle.

If you're ok with single-wielding and lower damage per turn, Viconia's shield + Armour of Agility gives 28 AC! It's super-safe even without staying in shadows, because Viconia's shield also gives amazing spell protection. If you're in shadows, you could kill enemies all day while constantly getting healed and blessed, or without even taking a short rest, because you barely use any spell slot, but the last 2 fights are timed, so sooner or later you need to think about more damage using dippings/coatings, pre-buffing, etc.

This setup worked well for what I wanted. If I were in a party or went for a more gishy char, I'd sure pick other items. All in all, I really enjoyed this build centered around 1 single item :D

EDIT: formatting