When you throw darts with the crossbow equipped, its special effect takes place. You just use the darts like you would any other throwing weapon. IIRC every shop should have at least a couple darts for sale.
But how to use darts during boss fights, specifically the King fight? I tried the Crossbow once as the Freerunner and it ended miserably because I ran out of my six darts quickly after the fight started. It felt like I was using a Spirit Bow but with very limited ammo. Surely I'm missing something here?
You also don't need all that many to be helpful. Set him on fire, poison him, maybe blind if you fancy, but then focus on minions (or whatever else damaging you have for the last phase, then invis until deferred happens)
That's my usual strategy w/ other weapons, but the crossbow kinda sucks as a melee weapon even after upgrading it to like +5 or something, so I can't even clear the ghoul swarm with it in melee.
Fair, that's where the elixir of toxicity (and optional stamina pot) come in during the bossfight. And in normal game, that's where I disengage when faced with 3 or more ghouls