Ok, so I gave it a go. It's not perfect and I'm bound to have missed something but we'll see.
My first thought was that summoning a creature means that you now have two characters running around to control which takes time away from other players and my second thought was that if you get hit/damaged while your demon is running around then what happens to the summoning spell. My final concern was to make it possible to bind the demon quickly but it still be a powerful spell i.e. not easy to cast.
So working with Ophidimancer's suggestion that they start with an extreme consequence like 'PC is my master' (the demons have previously been bound by you, you know their name, you've been given power over them etc) I came up with this idea.
You cast the spell, it takes an exchange or two but that summons the creature AND creates a ward around you. It's a 8 shift block that lasts the duration of the scene, as does the summoning. The summoned demon appears in the same zone as you and you basically control it like you would your normal character. Every exchange the demon tries to escape control, rolled as a mental conflict between you and the demon, it's conviction attacking your discipline.
With the build below you can:
Handle a summoning ritual of up to complexity 16 without preparation.
Complexity = Lore (6) + refinements (4) + focus item (6)
Control up to 10 shifts of power each exchange, 12 if your demon might kill things. With a few aspects invoked/tagged and a source of power (sacrifice or sponsor) you could technically cast it in one exchange. Otherwise you can cast it in two or three exchanges depending on how much mental stress you want to take.
The spell is:
Demonic Summoning: Complexity is 16 Shift Ritual
Barrier - a 8 shift ward (block) surrounding the caster only (using focus item)
Summoning complexity - 8 shifts
--18 Refresh--
[-3] Thaumaturgy
[-3] Sponsored Magic – Faustian Bargain & Hell-fire channelling
[-2] Lawbreaker (First Law) (+2 to casting roll for casting spells that will result in breaking the First Law)
[-5] Refinements (specialisation bonuses)
+1 Ward Control (Bonus for Thaumaturgy)
+2 Ward Complexity
+3 Summoning Control
+4 Summoning Complexity
[-4] Item of Power - Demonic Chain.
Demonic Chain
This item, granted due to a demonic bargain with a Lord of Hell, allows the wielder to summon demonic forces to his aid. It's a long, heavy chain that can be wielded like a whip or flail, covered in vicious barbs and engraved with demonic runes that hunger to taste blood. The weapons true strength, is in it's ability to create binds and wards. At a command, it can bind an opponent, causing him to writhe in pain as if he were wrapped in barbed wire. A second command can cause it to form a perfect, solid circle around the wielder; spinning and floating in mid-air as a barrier forms, or wrapping around the wielder as armour.
Weapon: 3, counts as Unholy, unbreakable
[+2] One-time discount
[-1] Blood Thirsty: When swung in keeping with it's purpose, it grants a +1 to wielders weapon skill.
[-1] The PC can summon the Demonic Chain to his hand with a thought
[-4] Refinements (additional focus item slots, on top of ) which are spent as:
[6 Focus Slots] Focus Item: +6 to Complexity (Summoning), +2 to Control (Summoning)
[2 Enchanted Item Slots] Enchanted Item: Fantastic (+6) Defensive Block 3 times a session.
[2 Enchanted Item Slots] Enchanted Item: Superb (+5) Offensive Block, lasting for 3 exchanges, once a session. Uses Discipline to target, opposed by Endurance.
Total refresh is -17
--60 Skill Points--
Fantastic (+6): Conviction, Lore
Superb (+5): Discipline, Athletics, Endurance
Great (+4): Alertness, Deceit, Weapons
Good (+3): Presence, Rapport, Resources
Fair (+2): Contacts, Driving, Scholarship, Stealth
Average (+1): Burglary, Guns, investigation, Survival
and some sample demons:
This one is based on the Scarecrow which you've mentioned.
High Concept: Fear Demon
Other Aspects: Nightmare Incarnate,
Skills
Alertness: Good (+3)
Athletics: Good (+3)
Conviction: Superb (+4)
Deceit: Great (+4)
Endurance: Good (+3)
Fists: Great (+4)
Intimidation: Superb (+5)
Might: Good (+3)
Most other skills default to Average or Fair.
Stunts
Smell Fear (Investigation): +2 to Investigation rolls to home in on or locate a source of fear.
Worst Nightmare: +2 to intimidation
Powers:
[–2] Glamours
[-1] Emotional Vampire(Fear)
[-2] Ranged Incite Emotion
[-4] Psychic Shapeshifting - always looks like the viewers worst nightmare, skills don't change.
[-8] Physical Immunity - Can't be hurt by anyone scared of him.
[+5] The Catch (Stacked) is anyone not scared of him.
[–1] Claws (in any form it takes)
[–1] Spider Walk
[-1] Wings (not always visible, depending on the viewers nightmare)
[–4] Supernatural Strength
[–4] Supernatural Toughness
[–4] Supernatural Recovery
[+3] Catch is holy items and expressions of genuine faith.
Total refresh is -25
This one is just a melee monster
High Concept: Demonic Squid Pig
Other aspects: Bestial Rage, Blind, Heartbeat Hunter
Alertness: Average (+1)
Athletics: Average (+1)
Conviction: Great (+4)
Endurance: Superb (+5)
Fists: Superb (+4)
Might: Superb (+5)
Other other physical skills default to Good, the rest to Fair.
Stunts
Sensitive hearing (Alertness): +4 to Alertness when listening for heartbeats if there's no other loud, distracting noises around.
Tentacled Monstrosity (Might): +2 when using Might to grapple.
Powers
[-1] Aquatic
[-1] Supernatural Sense
[-6] Mythic Strength
[-6] Mythic Toughness
[-6] Mythic Recovery
[+4] Catch is holy items, expressions of genuine faith, if subjected to extremely loud noises.
Total Refresh Cost: –16
And this one is so that I'm not just perpetuating the 'angry demon' stereotype
High Concept: Demonic Scout
Other aspects: One head two bodies, not a fighter
Alertness: Superb (+5)
Athletics: Great (+4)
Burglary: Fair (+2)
Conviction: Good (+3)
Stealth: Good (+3)
Survival: Fair (+2)
Other other skills default to Average.
Stunts:
[-1] Fear-fueled strength: When either body is in danger, it gets +4 to conviction as long as it doesn't attack.
Powers
[-1] Claws
[-1] Diminutive Size
[-1] Spider Walk
[-1] Cloak of Shadows
[-1] Psychometry
[-3] Broad Supernatural Senses
[-2] Greater Glamours (depicting holograms)
[-2] Inhuman Speed
Total Refresh Cost: -12
Note: While the 'head' (a vile rat-like thing with huge, bulbous eyes and vicious fangs) goes off to scout, the 'body' (imagine a TV made of flesh, covered in tiny mouths with one large mouth for a screen, inside of which is a pool of swirling ectoplasm) shows you everything the head sees, using glamours to provide a picture and the mouths for sound.
Hah, beaten to it by Ophidimancer
Well if we all try I'm sure we'll find a reasonable version. Also, without lawbreaker there'd be more refresh available for non-demonic spells