Stena takes the arrow in the shoulder and drops the halfling to follow the others and growls,

"Keep moving."

She'll then pull her longbow out and squints back into the darkness, and readies and attack for the first drow that enters her vision (90').

Readied attack: Attack: 27 Damage: 19

Stena slips back closer to the web intersection and waits, Kolose and the halfling hobbling farther away. Galingal hunches against the web, but the crossing strand is far beyond his vision.

Stena sees the drow approach, peering in the direction her companions went, their eyes passing over her. She draws back the arrow and sends it arcing through the darkness where it takes the nearest drow in the notch of his collarbone and he drops in a heap, blood spraying the web as he topples over the edge, the silk tugging at him briefly before it releases and he plummets into the darkness below. The other drow crouch one peering in her general direction as the remaining two begin casting spells, and a handful of purple glowing lights wink into existence strung above the silken strand, illuminating the backs of Kolose and the halfling, as well as Stena. However, at nearly the same time, a sphere of darkness surrounds the drow on the crossing web strand, obscuring them from Stena's view.

((How far back towards the drow would she need to move to get out of the light again?))

The strung the cluster out roughly where she was back to the extent of their sight (at the time) towards the others, it would be ~40 ft for Stena to retreat back towards the others and get out of the light, but only ~20ft to get back into darkness if she moves towards the drow.

Stena will move out of the light and hold a shot for anything the pokes its head out of the darkness.

Attack: 23 Damage: 8

Stena hurries forward beyond the soft glow of the drow-lights and holds still, the web slowing its swaying beneath her as her companions move farther off. The sphere of darkness holds still on the crossing strand above her as she holds an arrow drawn, eyes scanning for a drow getting brave. Her arm hasn't even gotten tired when a drow head pokes out, looking towards the illuminated strand behind her.

((He can't see her, so she'd have advantage.))

((Which I see you did roll with.))

The arrow hits him in the mouth, teeth fragments spinning into the dark as he curses and drops back into the sphere of darkness. The sphere begins inching slowly forward, but no more drow heads poke out.

Galingal decides fighting something he can't see isn't going to get him very far and moves to catch up with the others and keep moving. Hopefully the spiders would deal with drow.

He dodges as he moves.

Stena watches the black sphere move along for a moment then notes that, from her position, she can clearly guess where the middle of the sphere would be. Knowing that whoever is in the middle also is walking on the thread makes it even easier.

She fires into the sphere's center.

Attack: 9

Damage (w/da): 8

Stena hears a startled yelp, but it's more one of surprise than pain, as the sphere starts moving a bit more rapidly along the strand of silk. It is now positioned above the strand she is on, but 20 ft up, and 50ft away.

Stena isn't pleased she didn't connect and sight on what she supposes is the middle again and looses, walking slowly to keep up with the sphere as it goes.

Attack: 10

Damage (w/da): 4

The web Stena is standing on sags, then rebounds as a quaggoth drops down on the far side of the sphere of darkness and begins moving in her direction, sniffing the air.

Stena turns and fires on the quaggoth while moving away from it.

Attack: 13 Damage: 11

((Moving into the light?))

Retroactive hunter's mark: 1

Stena falls back slightly as she puts an arrow into the quaggoth, it looks around in anger after getting hit, before ambling along the web in the direction the arrow came from.

((Stena has a bit less than 20ft until she's back in the light of the drow spell, and the quaggoth has gained on her, now also about 20 ft away. The sphere of darkness has stopped above the silk intersection, and the drow within seem to be waiting for the quaggoth to track down the unseen attacker.))

(How close is Galingal to Stena?)

Galingal isn't sure, he can't see Stena, but he's 40 ft from the edge of the area lit up by dancing lights, and they extend for another 60ft, so she's either within 40 ft and invisible to him, or beyond 100ft and in the darkness beyond his darkvision range.

Galingal continues back towards the others, keeping a watch over his shoulder for the drow, but also for spiders, and will assume Stena is okay at this point. 

While dampened by distance, Galingal and the party would have felt the strand they are on dip recently. The more perceptive or analytical among them may have surmised that drow, quaggoths, or spiders have dropped onto the strand behind them.