Sons of Perdition

Now I've got an interesting assignment for you. There was a demon summoning incident right here in the capital.
The culprit is from a supposed secret society named Sons of Perdition.
They are young nobles who all claim that they were simply playing dark god worshippers to shock others
and have run.

A maid walked into a room where they were having a party. As she says, one of them uttered a spell,
two demons appeared in front of him and flown towards her. She ran for her life and notified a guard who called us.
My team killed the demons and arrested all those guys who were still found in that room,
seemingly dead scared.

The punishment for summonning demons is death. Summoners know that
and they always do everything they can to evade justice — summon as many demons as they can
and flee as fast as they can.
That young man, however, didn't even make any effort to flee,
and certainly didn't summon any more demons. And he was surprised that he now faces a death sentence.
And that makes me uneasy: eyewitness testimony points at him, but he doesn't look anything like
any summoners any of us ever faced.

As I recall, there was a dark mage who earnt clemency by revealing the location of their order's hideout
and helping us enter it to purge it, and his sentence was commuted from death to life in prison.
He's probably serving his sentence in the Dead Rock island prison. Since he has first-hand experience
with summoners, maybe he can tell us how to prove that guy's guilt or innocence.

You should also want to talk to that guy, he's in the detention cells now.

$supposedSummoner is indeed in a detention cell, guarded by two crusaders. He will maintain his innocence and tell you it was a game, until his made-up nonsense “spell” seemingly summoned demons for real:

I swear I never summoned any demons and I have no idea how it's actually done!
My friends and I just made up that game, a secret society of Sons of Perdition.
Walking around in black robes, shocking the elderly by saying "Dark gods help me!",
decorating our room with bizzare antiques, you know.
We were having one of those "dark god worship" parties when a maid walked in,
I wanted to scare her for a prank, raised my hands and said a nonsense "spell".
Except it worked too well: two demons actually appeared and went after the maid...
We were as scared as the maid, I tell you. She ran, we remained petrified
until crusaders came in and arrested us.
I didn't want to actually sommon demons. I'm so glad no one died and I'm sorry it turned out this way.
We'd all been saying that nonsense "spell" a lot and nothing happened. I even repeated it here
to see if I might be an accidental summoner, but it does nothing.
If you get me and my friends acquitted, I swear we will never play any such games again!

Now all you can do is to go to the prison where the former dark mage is serving his sentence. The Dead Rock isle is way to the North but during this quest you can mention it to shipmasters in the docks and they will take you there for $fee.

It's impossible to accidentally summon demons. The summoning spell is a secret only revealed by dark gods themselves.
In our order, we had twenty members and only five of them were summoners.
I spoke to Gidrykh through his shrine and he refused to make me a summoner. For the better, I guess...
When those summoners started planning a massacre for the glory of the dark god,
I realized that my quest for power went in a completely wrong direction. On my next trip to the town
I turned myself in to the crusaders and helped them plan a raid on our cave.

Anyway, if that boy didn't summon demons, how did they get summoned then?
You say they were collecting antiques... one favorite "joke" of summoners was to enchant various items
to summon demons when handled and leave them for people to find.
Maybe they were unfortunate to get such a cursed antique. I'd ask his friends what they were doing 
when the demons appeared and inspect the room.

There is no ship back from the prison island, but the warden will helpfully provide you with a scroll if you can’t teleport on your own.

Back in the capital, share the information with the head of crusaders and get his permission to inspect the room. Then go to $mansion and ask the maid for directions.

In the room, you will find an assortment of clutter, including a grinder. When you activate the grinder, you will get a menu with “pick up” and “grind” options. Try to grind it and two demons will appear. Kill them and take the grinder to the quest giver — you have found the cursed item.

Then go to the $supposedSummoner to bring him the good news. He will mention that they got the grinder from an antique shop for its impressive look, his friend fixed it and was grinding spices for mulled wine with it at the party when the maid walked in. $supposedSummoner’s spell created an impression that he summoned the demons, but it was merely a coincidence.

Note: after the quest, the former dark mage will be released from prison and granted a position in the crusaders order. He will give you his book on demonology as a gift if you visit him.