During your Dark Urge playthrough, a weird little guy will show up. He'll claim to want to help you discover your true self, and you might be tempted to take him up on that offer, especially if you think he can help you control your urges. It turns out though that he isn't here to help you control your urges, quite the opposite, he's here to unleash them upon the world. But who is this weird guy? We take a look and find out.
SPOILERS FOLLOW FOR THE DARK URGE
Who is Sceleritas Fel?
Sceleritas Fel is a Goblin in a Top Hat. He's clearly evil, but he's also clearly indebted to Bhaal in some way, given how much he constantly brings up the Murder God. As you learn later in the game, you're a Bhaalspawn, a child of the Murder God, and he's basically your butler. He'll appear throughout the playthrough and offer you bonuses and extra powers if you choose to murder everybody that he wants you to murder.
It's also possible to completely ignore him and let him just be annoyed at you for not living up to your father and to your true nature, and perhaps this is the best playthrough for you if you want to see the 'true' path through Baldur's Gate 3.
Later in your game, you'll have the option to kill Fel, assuming that you took the darker path of the two paths offered to you as The Dark Urge. You'll enter your Slayer Form and stab Fel before you're offered the power of Bhaal (Power Word: Kill, a one-time use spell that instantly kills an enemy with absolutely no saving throws whatsoever). It's a moment of catharsis, and one that feels like it was a long time coming, but it's also a representation of you falling to your darker desires, so be wary!