A crossover version of Shadowhunters + the wider Shadow World mythology with Arthurian lore actually fits extremely well because the universe already treats old myths as distorted history.
Myrddin Emrys / Merlin Concept
Core Identity
Myrddin Emrys, known throughout history as Merlin, is one of the oldest living warlocks in existence — potentially older than even Magnus Bane depending on continuity.
He is:
The architect behind many magical institutions.
The hidden founder of Britain's magical governance.
A neutral power respected by Shadowhunters, Downworlders, Seelies, and even certain Greater Demons.
Feared because he understands all forms of magic:
Demonic
Angelic
Faerie
Necromantic
Ancient druidic ley magic
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Parentage Options
Option 1 — Demon Father + Human Mother
Closest to standard Shadowhunter warlock lore.
Father
A Greater Demon Prince — possibly:
an original demon of knowledge,
prophecy,
or chaos.
This explains:
unnatural magical reserves,
immortality,
cat-like warlock marks,
prophetic visions,
ability to manipulate dimensions.
Mother
A human druid priestess from ancient Britain.
This gives Merlin:
connection to ley lines,
old Celtic magic,
empathy toward mortals.
This version makes him essentially:
> "The first truly great warlock."
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Option 2 — Demon Father + Seelie Mother
Probably the strongest mythological version.
Why it works
It explains why Merlin feels:
neither fully demonic,
nor fully mortal,
but deeply connected to Faerie.
He would possess:
glamour magic,
true-name magic,
emotional enchantment,
time distortion abilities.
This version could make him:
partially immune to iron,
capable of walking between realms naturally,
tied directly to Avalon/Faerie.
It also creates direct links to:
Morgan le Fay
The Lady of the Lake
the Seelie Court.
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Morgan le Fay
Best interpretation:
Morgan le Fay = Ancient Seelie Royalty
Not evil — just inhuman.
Possible roles:
Merlin's half-sister,
former lover,
magical rival,
or co-ruler of Avalon.
She could be:
a Seelie princess,
future Seelie Queen,
or the original bridge between Faerie and humanity.
Her philosophy:
> "Human morality is temporary. Magic is eternal."
That contrasts Merlin's more balanced worldview.
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The Lady of the Lake
This title works better as a mantle than one person.
Possible Interpretation
The "Lady of the Lake" is:
the guardian of Avalon,
keeper of magical artifacts,
protector of Excalibur,
and warden of portals into Faerie.
Nimue
"Nimue" could be:
the current Lady of the Lake,
Merlin's daughter,
apprentice,
or lost lover.
A powerful angle:
> Nimue imprisoned Merlin not out of betrayal, but to save the world from him.
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Magnus Bane Relationship
Two great dynamics exist:
Mentor Version
Merlin trained Magnus centuries ago.
Magnus:
learned diplomacy,
emotional restraint,
and higher-dimensional magic from him.
Merlin would be one of the few people Magnus genuinely respects.
Their relationship could feel like:
old friends,
exhausted immortals,
or surrogate family.
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Disciple Version
Even more interesting.
Magnus discovers:
Merlin is alive,
hiding in Britain,
and vastly beyond modern warlocks.
Merlin sees Magnus as:
> "The closest thing this age has to an heir."
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21st Century Role
High Warlock of Britain
Perfect role.
Unlike Magnus:
Merlin operates politically.
He governs magical balance quietly.
He would likely:
mediate Shadowhunter/Faerie conflicts,
regulate portals,
suppress demonic outbreaks,
maintain ancient wards around Britain.
London could secretly sit atop:
dragon ley lines,
ancient demon prisons,
and Avalon gateways.
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Harry Potter Crossover Integration
This actually meshes shockingly well.
Merlin could secretly be:
founder of the modern wizarding world,
creator of the Wizengamot,
architect of Hogwarts' deeper wards.
Titles
Headmaster of Hogwarts
Supreme Mugwump of the International Confederation of Wizards
Chief Warlock of the Wizengamot
Grand Sorcerer of Britain
The Order of Merlin being named after him while he is secretly still alive is very fitting.
Even funnier: He occasionally receives his own Order of Merlin awards under fake identities.
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Power Level
Merlin should feel like:
a once-in-history magical anomaly,
not just "stronger Magnus."
Abilities
Reality warping
Time dilation
Soul magic
Realm creation
Dragon-binding
Dimensional seals
True-name magic
Necromancy
Angelic rune understanding
Demon language mastery
Potentially the only being capable of:
fighting both Greater Demons and Archangels,
or sealing entire dimensions.
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Best Overall Version
The strongest lore combination is probably:
Merlin
Half Greater Demon
Half Seelie/Druid-blooded witch
First Great Warlock
Founder of Britain's magical systems
Secret ruler/protector of Avalon
Mentor to Magnus Bane
Rival/ally of Morgan le Fay
Guardian of balance between Shadow World and Faerie
That version feels mythic enough for both universes without breaking either setting.