Well they both have valid points. All you have to do is convince one of them to back down and get whatever Sullinus knows, or thinks he knows, out of him. Hopefully it will be useful.
First steps first. "Sullinus knows something," you stage whisper to Freyja, leaning a little closer to her to try and further the illusion of subterfuge, "he may not be willing to share what he knows. Either he's afraid of the repercussions or he's protecting someone." Freyja gives you a distinctly unimpressed look. Sullinus arches a brow at you.
"I am willing to share what I know but there's no point if I am not around to help you deal with what I think is happening." Sullinus says, trying so hard to be reasonable and calm that he's coming off as being at the last fraying ends of his temper. You doubt that's the case.
"Look, Sullinus, Freyja, the way I see things there's no harm in all of going to check on the remaining unaffected mages while Sullinus explains what's going on." You wheedle, wrapping your arms around their shoulders in the friendliest way you know how. Sullinus squirms uncomfortably. You friendship hug him harder.
"But the obelisk...."
"It'll be fine." You assure her glibly, flapping a hand nonchalantly. "My obelisk. And the only way anything bad happens to it is if a super Knight walks out of Order itself and carts it away. Not gonna happen since Sheogorath is still Sheogorath rather than Jyggalag."
"You know at least one part of that is not at all reassuring." Sullinus says. "And get off." You release him. Maybe you overestimated the power of your friendship hug.
"If something worse than you and your minions walks out of that obelisk I'm blaming you." Freyja whispers at you and she thumps the middle of your breastplate with the rim of her shield. "Got it?"
"Yep, got it. Now how about we start walking and Sullinus starts talking?" You suggest amiably. Freyja starts walking in the general direction of the more expensive area of the city. That's probably where the rest of the list lives. Make sense, most of the rest of the incidents have been in the poorer areas.
As you walk Sullinus talks.
"About nine months ago there was a new face at the guild hall, a Dark Elf called Medresi. She wanted a recommendation, like they all do, and help with her research. She was something a summoner and she was working to master the spell to conjure a Spider Daedra. The rest of her time she spent researching all the kinds of spiders in Tamriel. Frostbite spiders in particular caught her attention and she tried to have some eggs shipped in from Skyrim. Adrienne put her foot down."
Oh you can see where this is going.
"She.... She was very driven, Adrienne suggested that she might have more luck with our colleagues in Chorrol. There was an argument." Sullinus trails off. "Adrienne is... can be abrasive and harsh. She cited previous research into attempting to compel creatures of Nirn as the masters of conjuration can with summoned daedra and the like, with rather more force that perhaps she should have. Medresi didn't take it well....
"I tried to talk to her after but she didn't want to know. The day after she was gone. We assumed she'd gone to Chorrol to spend time with more dedicated summoners. It seems now she has not." Sullinus stops speaking and you arrive in front of a large, sprawling house, shutters painted bright blue to match the door and the glass in the windows is some of the highest quality you've seen. This place must be expensive.
"And no one thought to mention this when the spider infestation started!" Freyja's furious and Sullinus shrinks closer to you.
"I-I didn't know there were frostbite spiders in the city until you told me." Sullinus quavers, practically hiding behind you now. Freyja breathes out hard through her nose before knocking on the blue door hard enough you're surprised it doesn't cave in.
No one answers immediately. Freyja keeps knocking and you scout around the exterior of the building.
You find a hatch at the back. It's open and you think you see a flutter of fabric vanishing down it.
[ ] What do?
*grumbling* stupid idea/challenge from stupid D&D session WHY AM I DOING THIS??