Reprieve!
Metron needed to make a successful Concentration roll in order to have cast that spell!
Ease Factor 12 (Hard, Dodging)
Sta +1, Concentration 3 (spells)
And I rolled a die at home. He failed, and he rolled a simple bbotch, the spell simply fails.
Octavian, your Parma is still good. Let me catch up on the other posts now 
The spirit was purposfully faking you out. Metron's MO involves confusion and befuddlment.
You cannot see either Metron or Octavian, but you hear them.
OOC: Gah! I had login troubles for a few days, and a LOT happens in the interim.
IC: Vares tries to calm the situation between Octavian and Ludoviccio. "We were all exploring different questions and threads in the city. Curiosity is a common attribute of Magi, after all--and we are not required to constantly inform others of our whereabouts as long as we respect Barcelona's rules and the Code.'
The sound of the explosion, however, causes him to cast his own senses outward, searching for the source. He relates to Ludo exactly what appears to have happened while Ludo, wisely, brings up his Parma.
The summoning of the retinue is something else that Vares is quite familiar with. He does not intervene as they gather. Ludoviccio has done nothing, in his mind, to indicate a clear violation of the Code--and so to hinder him would qualify as a diminishment of Ludo's own magical strength. A point he will cite for Octavian.
"Alexandro," he tells the guard. "Octavian may require our assistance. It would be better if we investigate than remain here." He chooses to use persuasion rather than threat to gain compliance.
His attempt at grabbing Metron's arms failing, he lands crouched in the dirt. He narrows his eyes into sinister green slits towards Metron. Uncharacteristically he speaks up to his opponent. His tone more facetious than normal, almost taunting. "It has been a long time Metron. What fascinating eyes you have now. Even more fascinating than Relegare's."
(in the old Iberia book she had Presence 3 (alluring eyes) back when you had specialities for characteristics)
[color=green]Metron: [color=red]Releganta! Ha! I pity that ignorant whore. Rasus fed falshoods to each of us, but she got the worst of it. Her very Gift is a lie! And as for me, I have learned that his fictions were actually a twisted misunderstanding of the truth. I have become the living embodiment of the Flaming Shadow!
And his eyes burn with blackness. Then he hears Inigo shout.
In Ars Magica, you only roll Initiative once (Fast Cast events are handled separately).
The order is still as follows…
Inigo
Octavian
Metron
(Note to Falls, Octavian is still invisible so the only indication he has of his presence would be the sound of his voice for the time being).
Can I see Metron now, or is he invisible as well? If I can’t detect where he is well enough to cast a spell on him, I will take a round to cast Vision of Heat’s Light on myself.
If not, and if I perceive no immediate danger to myself or my sodales, I will multicast Wizard’s Icy Grip on Metron.
First Casting
Casting Total: 73 (Die roll 9, Casting Total with virtues, mastery, & talisman +53, Voice and Gestures +2, Aura +10)
Penetration Total: 48 (73, -30 spell level, +3 Penetration score, +2 Spell Mastery)
Damage: 22 (Die roll 2, +20)
Second Casting
Casting Total: 70 (Die roll 6, modifiers as above)
Penetration Total: 45 (see above)
Damage: 29 (Die roll 9, +20)
Third Casting
Casting Total: 66 (Die roll 2, modifiers as above)
Penetration Total: 41 (see above)
Damage: 30 (Die roll 1, then 5, +20)
If using Confidence will shift things significantly, I will do so.
First Casting
Second Casting
Third Casting (rolled a 1 for damage)
Damage reroll (from rolling a 1)
If it looks like Inigo can't see Metron, Octavian will try once more to grapple him and then end his invisibility spell so he reappears. He will then inform Inigo that he's holding Metron.
Attack Roll: invisiblecastle.com/roller/view/1932247/ Total 14 I miscalculated my bonus as I forgot my specialty in grapples. I will spend additional confidence if necessary to hit.
Metron suddenly and immediately melts away into nothing. That's what it looks like anyway. He is gone. His house is still in flames, and he left behind the bag he dropped.
Vares, you were ordering the men to head out, and I presume Ludovicio has headed out on his own. But before you take the lads out, what do you do with the child and the woman tied to the chair?
(OOC: Would this have happened before or after Inigor and/or Octavian's actions?).
He scans the area quickly for any signs of the magus. He reappears if he has not already so that he is visible to Inigo. "It is likely he will try to make an escape. I ordered the others to remain once my suspicions of Metron's presence here were confirmed." He approaches the bag and picks it up.
OOC - Inigo keeps using the wrong spells 
Metron is immune to cold and he has a ward against heat & flames. And he can't see either of you. My best guess at a reasonable cource of action would be for Inigo to use that spell that covers and area in ice (or is that this spell? I get them confused). Octavian would fast cast Wizard's leap to get out of the way, and the bag would be frozen in ice. But regardless of how it goes down, Metrom has the power to instantly dissolve into nothingness, specifically in order to avoid harm or hazzard.
The bag contains Metron's Lab Texts from the past few years, including his Magic Realm Transformations.
Vares and the troops have left the guest house. I am waiting on Arwan to tell me if they left the woman there or if they took her with them. But he cannot undo the decision to leave. I am already exploiting his tactical error.
OOC: Vares' instructions to the guards were chosen to minimise apparent conflict, since Ludoviccio appeared determined to leave, and sparking yet ANOTHER fight would be counterproductive.
I didn't realise that the woman and child were tied up. Are they in the main room, or a smaller room?
Vares would not leave them there without some guard to watch over them, and would cast a concealment spell to keep them from being spotted readily ("Aura of Inconsequence"). Someone searching for them consistently would probably be able to know that they were there, but anything short of that, they'd be overlooked.
Okay, you left poor Roger behind then.
Octavian and Inigo are sorting out the confusion of what just happened when the entourage comes around the corner. Octavian slaps the front of his head with an open palm.
Then BOOM, the guest house explodes in black fire. Metron multi-cast "Ball of Abysmal Flame" as he retreated through the Portal, the blackness is his exaggerated sigil. That and a remnant Taint of Evil.
The Portal is gone.
The woman is dead.
Roger crawls from the wreckage holding onto a burned and screaming child, then collapses dead.
On the bright side, the demon shows up and it is utterly ineffective. A Vessel of Iniquity, representing Horror and Despair, manifests in the road where you all are. It is a shapeless horror without definable dimension. It exhudes a power that causes panic and hysteria, but it bounces off of everyone's Parma and the grogs all make successful Brave personality rolls to resist it.
The village children are immune, because the Demon and Metron both share the same weakness in that they cannot harm children.
But the few surviving adults that remain in town, they have not acquired a might score as the children have. They go nuts.
One man and his wife had been cowering in their basement for years, and they were only just now begining to feel safe. But then he became convinced she was plotting to kill him, so he snapped her neck.
And it gets worse. But my morbid creativity is not up to the task. Use your imaginations.
(OOC: If I may, I'd change the description of Octavian's actions as below)
Upon sighting the group approaching, his eyes widen and he immediately sprints back towards the guest house. He ignores the magi and custos as he runs past them, occasionally casting wizard's leap to hasten his speed. He stops in his tracks to shield his eyes from the explosion. Spotting the demon from the corner of his eye he turns. His eyes glow malevolently towards the creature as he prepares a spell, reaching into his sleeves for pawns of vis.
Done deal.
Don't panic guys, Metron may be attuned to magic, but the demon cannot be. You are in the Magic Realm, it is at a great disadvantage (which is why the shadow spirits he was using are Magical).
Octavian is closes, I am giving him the first shot at it. Followed by whomever replies next.
On the other hand, yeah, you should panic. I have no idea how you guys are gonna get out of this 
(OOC: Marko, does the contents of the bag (lab texts written by Metron) count as a penetration modifier for the demon since it has some form of connection to Metron?)
OOC: Having just noticed how weak vis is for boosting spell casting in 5th ed, Octavian will try to see gauge the strength of the demon first. He casts Demon's Eternal Oblivion PeVi 10 on the demon)
His casting roll is:
invisiblecastle.com/roller/view/1933318/ +1 for Loud Words + 1 for Exagerrated Gestures = 14
His Penetration is 3 * by either 1 or 3 depending if the lab texts would count as an arcane connection. Will blow confidence if it helps.
He lifts his left hand to point at the demon. The creature will see a murder of crows appear to emerge from his billowing sleeve and fly towards it to to peck at it to reduce the magical might.
[OOC: I was waiting to hear the guard's reaction before exiting that room and taking survey of the situation from the stoop of the door there. I also have second sight, so I'm curious if I may have had a chance to spot/intercept Metron as he was fleeing directly through my position to my understanding? Really my next post was going to be a 'standing imperiously on the stoop as he looks around and orders spirits' blah blah. If not, you don't need to explain how... I'll just be more intrigued than before. 
]
[ OOC: Assuming things flow on, Ludo would have been standing right on the porch of the guesthouse when it detonated, so I'd think Ludo would be recovering himself right in that locale? Would he potentially have been harmed by the explosion? ]
You don't even know that there are lab texts in the bag yet, I was just letting you know you found something interesting. And no, there are no lab notes that would provide a Sympathetic bonus, and even if there were, you do not have the time.
Your roll of 14, +10 for the aura, equals 24, minus the spell level of 10 - 14, +3 for Penetration = 17;
Itt has an Infernal Might of 25, -10 for the Magic Aura, equals a resistance of 15.
You pierce its resistance and drain the thing by 10 points of Infernal Might!
LOL, you cant stand half in and half out of the dungeon. (Old D&D joke). You may have proceeded with the group or on your own, but the events described didn't go down until you had gotten about 250 yards away from the house and turned a corner. You prolly did stand on the stoop for a moment acting imperious giving orders to your spirits. Thats all cool. That lasts for about a minute. Then you moved along. There was nothing to see from that vantage point. You could have spotted Metron, but he traveled using a different manner, as he did not want to run into any more surprises.