I realize now my post didn't convey this (should have included calling out instructions to Thorn and Bryn, and then Bonus Action Dash withdrawal), but this is exactly Iasu's intent. Even if these dogs didn't come with a self-destruct protocol, this sort of skirmishing, hit-and-run tactic is Iasu's bread and butter. If allowed, will edit my post accordingly.
I think I generally had it that way anyhow, but didn't map out every mechanic of it. Here it is though, in my estimation:
In the complete darkness, Ranged Attacks would have Disadvantage at best (could argue that they would miss entirely, since Darkness is like Blinded Condition).
Just thinking it through realistically though, I think you CAN fight someone close-range hand-to-hand in the Darkness. It's arguable that that should be Disadvantage as well, but I don't want to go that far. There is a very small remnant of light from the great room. Plus when someone is right up on you engaged, I think both sides can use enough sensory to duke it out evenly. Generally.
So with your writing that Iasu went up until he could get an attack at targets, I felt like that meant he went right up to the 5' point blank range, and used the Close Quarters Shooter Feature for a normal shot.
Then you said Hide if possible, so that would suggest withdrawal a distance and Hide. Wrote it something like that in the narrative.
Technically I guess Iasu went adjacent to Hound 1 and then moved away. Could it have allowed a Reaction OA? Maybe. Would have to think about it. I didn't think about it very long. Figured a Bonus Action use of Disengage or that Skirmisher feature would keep Iasu safe, plus the dogs in that moment were laser-focused on Bryn-Thorn sandwiches. But I didn't map it out in Mechanics Blue to explain it all.
See what you think of that and let me know if you have any corrections needed, but I'm fine with saying Iasu made the close-quarters strike and then withdrew back to somewhere near the threshold of the hallway and the great room, and then tucked into the shadows.