How dangerous is Twilight to the person next to you?

Magical Twilight is an intensely personal experience. And yet it can be said to be magical energies raging out of control.
So do this magi out of control affect anything other than the Magus?
eg A magical aura forms within arm's reach of the magus' position, or something else?
What happens to someone who is touching the magus? Perhaps a custos or apprentice that supports a magus who stumbles as they botch a spell.

Generally, it is completely non-hazardous to people around you. The text at the top of page 89 of the 5th edition rulebook gives ideas - frequently magi fall unconscious or disappear into twilight. The final effect listed, of Flambeau turning into large fires which burn without fuel, could be hazardous to others.

In general, I play it as "Twilight forces you to react with your own internal reality, and you can't affect the external universe and it can't affect you" - given a maga who remains in the real world during twilight is immune to magic, mundane damage, aging and hunger.

[Of course, if you want to you can copy the Paradox mechanics from that other game initiated by Mr Rein Hagen, and have magical backlashes in reality or wizards disappearing into Paradox realms and their friends considering whether to jump in and rescue them or not.]

Interesting. Maybe I can still argue my character's back-story with the troupe.

I was thinking that the backstory for my new character would be stranded by circumstances with an old Bjornaer, who was offering to make my character her apprentice. Before this could officially happen, she botched a spontaneous spell and went into Final Twilight. With my character trying to physically support her.
Before Twilight fully grabbed her, she tried to console my character with "So long as I live, I look after you and keep you alive", then she transformed into a Great Beast and raced off. The appearance of a Great Beast attracted other magi, who claimed my character as an apprentice.

With that backstory I was hoping to justify my character having Death Prophecy (can not die while that particular Great Beast lives), Plagued by Supernatural Creature (the Great Beast has a compulsion to look after my character, though she lacks the human bit to fully understand what she is doing), as well as Warped Magic and Weird Magic.

Very reasonable, the way I see it.

I had a young forge companion (with latent magic ability) who was there when her master - the prima domus of Verditius - had a laboratory accident and fell in twilight... and her latent ability became a real gift..

Is there any advice on designing Bjornaer Great Beasts?
If my character is going to be Plagued By Supernatural Creature, I need to design it. All I had was that it could fly, and it was large enough to carry my character, in order that he has the personality trait "Enjoys Flying" +2. I mistakenly assumed the SG would fill in the details to best fit the saga.

What sort of (presumably chimeric) Great Beast would qualify?
Perhaps a griffon or Pegasus?

I guess I was assuming that the Bjornaer maga had developed a sanguine (I think that is the gregarious one) disposition as she aged, thus was seeking human company when she met my character in the backstory.

Has anyone used a Peryton in saga before?
I mean peryton could be a corruption of "Fera Donna" (which I think is Latin for "wild woman"). Or perhaps a corruption of "Ferra Donna" (if my Latin works would be "iron woman"), as I am suggesting my character's Warped Magic is randomly giving nearby objects a temporary but strong magnetic field.
Say a Reindeer heartbeast - female reindeers have antlers. Checking the, perhaps this Great Beast maga had a couple of twilight scars, say the heartbeast had the jaws and appetite of a wolf, and in both forms casts the shadow of the maga as she was just prior to initiation into the Heartbeast mystery.

The peryton idea just sort of unfolded in my head. I am too close to it. Have I made any obvious mistakes?

If the SG allowed you to design the Great Beast, so much more power for you!

Great Beasts are best designed if you have access to Realms of Power: Magic; it's suggested they should have Might in the 20-40 range, perhaps roughly equal to Heartbeast score x 5.

A heartbeast need not be chimeric to be large enough to carry a human; an eagle heartbeast that has taken half-a-dozen Size refinements should definitely be large enough.

Perhaps more importantly, a heartbeast need not have wings to fly. A horse, deer, or similar hooved creature might just be able to gallop so swiftly and lightly that it can do so even on air (as a Power). And there's no reason why a Great Beast who was once a magus would not still retain use of some of the magical items crafted before succumbing to Twilight and enabling it to fly even in beast form.

I should have said I have been required to describe the Great Beast my character has been "plagued" with. After all my character starts the game with years of encounters with the Great Beast, so should be in the best position to describe it. The SG will take that description and tweak the numbers.

It hadn't occurred to me that the ex-Bjornaer did not necessarily need wings. Now my inner munchkin is trying to run wild. Suddenly a flying lightning wolf, ala pops to mind.

A few questions though, as I don't easy have access to Bjornaer focused books.
Do Great Beasts age and die? I thought beasts of virtue generally did.
How many Great Beasts would there be in Mythic Europe?
What would House Bjornaer think of a character that is plagued by a Great Beast?
Would a Great Beast be a candidate for becoming a Familiar?

Magical beasts aging - according to RoP:M p56, a Summer magical beast has passed the natural lifespan of its species. So they do age and die, but can live much longer than normal creatures.

How many Great Beasts? This is never addressed. There could be a couple per major animal, or there could be as many as there have been seriously shapeshifting magi long in the past.

What would House Bjornaer think? That you deserved it. One looking to impress a Great Beast for house mystery initiations might choose to victimise you to win favour with the beast.

Would a Great Beast be a candidate for becoming a Familiar? The prouder and mightier the creature, the less likely it is to want to become a familiar. It is unlikely one of the Great Beasts would agree to being a familiar. Also, the higher the might (and most Great Beasts are might 25-40 IIRC from HoH:MC) the harder they are to bind. Finally, as some Great Beasts were originally Bjornaer magi, who can't have familiars, they may be magically incapable of forming a familiar bond.