They currently have 11 Refresh, and they'll either have 12 or 13 by the time the Knight shows up. From what I recall of there stats I have a Knight with Weapons 5, a Wizard with Conviction/Discipline Great, a Werebear who acts as the Tank offensively, and a WC Vamp who's stats I can't recall but he's fast, has Awareness as his Superb stat, and who I assume can put up a fight since he's a former marine but he hasn't shown off much.
I love the coin idea.
If an above Submerged wizard doesn't have Superb in either conviction or discipline, I'm assuming he's not massively combat optimized. In that case...
OK then.
High Concept: Fallen Knight
Other Aspects: Host of Ferrumiel, Master Swordsman, Arrogant and Cruel, A Bloodstained Past
Skills (38 skill points)
Superb: Weapons, Athletics
Great: Alertness, Discipline
Good: Endurance, Intimidation, Lore
Fair: Contacts, Empathy, Resources
Average: Conviction, Driving, Might, Presence, Rapport
Stunts
Calm Blue Ocean (Discipline): +2 to Discipline to control emotions
Swordsman (Weapons): +1 to Weapons attacks with swords
Wall of Death (Weapons)
Powers
Inhuman Speed [-2]
Inhuman Strength [-2]
Inhuman Toughness [-2]
Supernatural Recovery [-4]
The Catch [+2]: holy
Sponsor [-0]: take sponsor debt on Weapons rolls; agenda: to kill and corrupt
Ferrumiel's Coin-Sword [+0] - not an Item of Power rebate since he can summon it into his hand at any time
It Is What It Is: one-handed arming sword, weapon:2 (weapon:4 with his strength)
Indestructible
Soulstrike: allows the wielder to make mental attacks in melee, weapon:2 (no strength bonus) [-1?]
Hellfire Blast: allows the sword to be used for ranged attacks, up to two zones [-1?]
Stress
mental ooo
physical oooo(oo) Armor:2 bulletproof vest, Armor:1 if not wearing armor or when hit by stress that bypasses the vest
social ooo
Total Refresh Cost: -13
That gives him Fantastic physical defense, Fantastic mental defense against emotions (Great otherwise), and Fantastic attacks (physical weapon:4 melee or ranged, mental weapon:2 melee). He can shed a mild consequence as a supplemental twice.
Depending on how combat heavy your players are, you might want to give him Supernatural Toughness (physical stress oooo(oooo) Armor:2, and adding 2 to his total refresh cost) and give the sword True Aim (boosting his attacks to Epic, and adding one more to his total refresh cost). What I have above isn't that much stronger than a PC at your level, so since he'll be facing 4 PCs, a boost is probably a good idea. But I don't know how good your players' defenses are, so I'm not sure whether to add the True Aim or not.
EDIT: added True Aim suggestion, more aspects, Fallen's name, worn armor