Author's Note: So to answer some questions about this- yes Harry will get 'the best of both worlds' in regards to his magic skills. But in some unique ways and definitely not in a way that will have him steamrolling the world. If you cannot tell I am also setting up wizards like Dumbledore, and Voldemort, (And yes Harry) to be a Big Deal. And powerful enough to not be an easy thing to get around with limited spell slots.

Of course Voldemort is just a Bad Match-up for Theodore's talents. Versatility Theodore can outthink and overpower. But Versatility with Power and Cunning? Theodore has a problem with that combination. And to be fair so does literally everybody.
Bonus: Rune Magus: I won't address your whole review bc I like to save some surprises, and I don't have a very extensive profile, but you'll be pleasantly surprised because I am literally a pseudo-pagan archaeologist of Irish descent who loves their ancestors' lore to death. And I'm gonna hardcore mesh Earth and Exandria magic together in Harry. There's a reason I went with Exandria instead of the Forgotten Realms other than my love for Critical Role.

Chapter 2: Destiny

"I am not sure how much your strange magic can protect them friend. Truly do you have any spell equalling or surpassing the Fidelius? Already three Potter properties that were heavily warded have been destroyed in Lord Voldemort's hunt for young Harry," Dumbledore elaborated.

With a deep sigh Theodore, the name he had taken forty years ago when he had arrived upon this plane, contemplated his vast knowledge of the arcane.

As he had never managed to find a way to cast Wish and he was not a cleric that could beseech some deity...and likely one that could not reach this realm anyways, he came up empty handed.

Besides, as a Bladesinger his spells had never really focused on large scale warding. He was a mage of war, using spell and sword to slaughter the foes of all good beings.

And he was lacking in this war. So lacking. He had managed to duel Voldemort once.

The duel had cost him his right hand to Fiendfyre. Oh he was ambidextrous to begin with so he was not out of combat.

But it had been a blow to his ego.

Even though he had found ways to make his known spells counter the Killing Curse he had still been defeated. A Crown of Stars and a Mordenkainen's Sword to deflect the sickly evil green blasts had drained too much of his energy for him to cast enough other spells to close in to melee with the Dark Lord.

The closest he had come was getting a good solid Booming Blade in thanks to his speed with Investiture of the Wind.

And that had caused the enraged Dark Lord to let loose the Fiendfyre.

He could kill any five wizards in this world, except for the once in a generation wizards like Dumbledore and Voldemort it seemed.

The kind of magic they threw around just made his technical and intellectual skills irrelevant.

It was as if they were both Sorcerer and Wizard at the same moment. It was infuriating.

Perhaps if he had access to a library of his home realm's magic he could find ways around the problem, more situationally useful spells, magical artifacts, and such.

All he had were his family Moonblade, his bag of holding, and the Ring of Shooting Stars.

Well he had some other magical knick-knacks he had picked up in his adventures as well, but those were what he used the most. Oh sure he had used his Wand of the War Mage to pass as a normal Wizard his first few decades in the world.

But eventually he had disappeared into the Muggle world, where he met his beloved Rosemary.

He had one wish left on a Ring of Three Wishes...but he could not figure out how to turn a single Wish into a ward more potent than the Fidelius. He would instead give the ring to Lily for her to use in an emergency, that was the most he could do.

"No old friend, I cannot say I have a method in this realm of aiding better."

Theodore and Albus had known each other a long time, Albus had in fact been working with the Unspeakables to find ways to undo much of Grindelwald's damage and curses to various areas of Europe when Teo'drine came through the Veil of Death much to everyone's surprise.

Unfortunately they had also quickly discovered it was a one way trip, any who passed through could not pass back. And it was unknown whether it went to the same location every time or in fact portaled through a nexus.

And so Theodore had taken up a new name and in return for shelter and learning of the new world he found himself in had aided Albus and his Unspeakable allies in their secret quest to root out the followers of Grindelwald's influence across the Iron Curtain where it had secretly thrived.

"Albus...I...please protect my daughter, Petunia forsook us after dear Rosemary died. She, and now little Harry, are all I have left."

The bearded, aged wizard reached out and placed a hand on his old friend's shoulder as the half-elven man sat crying silent tears in his favorite leather armchair.

"I will be doing my best, Theo. Voldemort shall not get the best of us, and if young Harry truly is his downfall we have quite the road ahead of us."

Theodore grunted, "Power he knows not'. It almost has to come from my side of things doesn't it?"

"Indeed, from what you have told me of your world I think it would have to. Some kind of sorcerous bloodline from your elven half perhaps? Latent until mixed with the natural magic of James Potter's blood?"

"It's a distinct possibility," his fingers steepled together in thought, "A member of my family was once a Champion of the Raven Queen even. Though I know not how she could reach us here."

"The...grim reaper of your world?" Dumbledore inquired.

"Hmm not quite, imagine Hel from your Nordic pantheon with a bit of Thanatos from the Greek if I understand correctly."

"It's fascinating that in your world there were so many deities so closely involved with events. Why in this one I have yet to even find definitive proof of the most worshipped of deities."

"I think it is the preponderance of Muggles here, Albus. With the mages all locking themselves away in their own little hideaways the deities have to be just as subtle and distant or the Muggles would react poorly."

"As magic withdrew from the world so too did the gods...I cannot say it is not a theorem that has not been tossed around in a few circles."

With a chuckle Theodore stood to his feet, his complexion slightly renewed from the harrowed look he had previously had, "A good intellectual discussion always raises my spirits, you know me too well, dear friend. However I think it's time you head back to your school."

"Yes, as nice as your house is Cokesworth itself is a bit...mundane for a man such as myself is it not?" Albus chuckled at his own joke and politely turned to leave, collecting his multicolor umbrella from the stand on his way off.

Once Albus had departed Theodore hastily made his way to his study. With a gesture and slight use of Thaumaturgy a blackboard rolled down from the ceiling and he swiftly drew a tall rectangle lined in runes with a piece of chalk.

A few muttered words of Elvish and he was walking through a black portal into his personal demiplane.

In this demiplane he stored all his knowledge and research, from both Exandria and Earth. While he had never been able to copy new spells into his books on Earth, it seemed his magic was incompatible with how spells were created here, he had managed to store some scrolls during his adventures here that had allowed him to transcribe some more into his personal grimoire.

And an extra copy of Xanathar's Guide to Everything was never without its own usefulness. In fact study of that tome, along with a few others, had been how he had puzzles out the way to cast the Crown of Stars spell which he had used to great success against the Unforgivables in the past.

Of course without a scrolls with a perfect and complete incantation to transcribe such study had taken him much of 1964 to complete.

Having a Cruciatus Curse pierce your Shield spell once was enough to be determined to find ways to stop a recurrence.

His current project was a spell he had previously found abhorrent and which he had not even begun to study until his family was in immediate targeted danger.

"Soul Cage..." He knew he could use it to interrogate dead Death Eaters, and hopefully find Voldemort's weaknesses and plans.

Which he could use to protect his Lily, and little Harry.

There were no limits to which a de Rollo would not pass to protect their family.

If there were any demons he could cavort with in this realm Theodore would have called on them just like his ancestor had.

But instead, he was simply left with reverse engineering necromancy spells.