The Mage's Arrival in Slaver's Bay (Spoilers Extended)

Background

While discussing the Meereenese Knot, GRRM mentioned a character that hasn't arrived yet:

For example, I wrote three different versions of Quentyn’s arrival at Meereen: one where he arrived long before Dany’s marriage, one where he arrived much later, and one where he arrived just the day before the marriage (which is how it ended up being in the novel). And I had to write all three versions to be able to compare and see how these different arrival points affected the stories of the other characters. Including the story of a character who actually hasn’t arrived yet. – SSM, Asshai.com Interview with GRRM: 28 Jul 2012

and while you can argue that both Tyrion/Victarion aren't in Meereen yet, I think GRRM was being vague on purpose and was in fact referencing another character that is heading to Meereen: Marwyn the Mage. That said even if GRRM wasn't talking about Marwyn here, his impact on the storyline is still worth discussing.

and:

And there are also a number of characters that go towards Meereen and must be taken into account: they are Quentyn Martell, Tyrion, Victarion Greyjoy with his Iron fleet, Archmaester Marwyn of the Citadel of Oldtown and Aegon with his golden company. And also highlight the situation with Volantis, which has not turned out to be as expected. -SSM, Asshai.com Interview in Barcelona: 2012

and:

I have legions of secondary characters, not POVs but nonetheless important to the plot, who also figure in the story: Lady Stoneheart, Young Griff, the Tattered Prince, Penny, Brown Ben Plumm, the Shavepate, Marwyn the Mage, Darkstar, Jeyne Westerling. -SSM, A Winter Garden: 8 July 2012

Marwyn's Pre-Slaver's Bay

Just a quick catch up on Marwyn's place in the series:

• While we meet Marwyn when Sam arrives in Oldtown, he is mentioned much earlier as characters from AGoT (Mirri Maz Duur) and ASoS (Qyburn) both mention interactions with him.

"Marwyn, he named himself," the woman replied in the Common Tongue. "From the sea. Beyond the sea. The Seven Lands, he said. Sunset Lands. Where men are iron and dragons rule. He taught me this speech."

"A maester in Asshai," Ser Jorah mused. -AGOT, Daenerys VII

and:

Do you believe in ghosts, Maester?" he asked Qyburn.

The man's face grew strange. "Once, at the Citadel, I came into an empty room and saw an empty chair. Yet I knew a woman had been there, only a moment before. The cushion was dented where she'd sat, the cloth was still warm, and her scent lingered in the air. If we leave our smells behind us when we leave a room, surely something of our souls must remain when we leave this life?" Qyburn spread his hands. "The archmaesters did not like my thinking, though. Well, Marwyn did, but he was the only one." -ASOS, Jaime VI

• After traveling far and beyond (even to Asshai) he wrote a book that Rodrik Harlaw seems to find valuable that has information regarding "The Targaryen Secret/The Song of Ice and Fire":

When Marwyn had returned to Oldtown, after spending eight years in the east mapping distant lands, searching for lost books, and studying with warlocks and shadowbinders, Vinegar Vaellyn had dubbed him “Marwyn the Mage.” -AFFC, Prologue

and:

"Nuncle." She closed the door behind her. "What reading was so urgent that you leave your guests without a host?"

"Archmaester Marwyn's Book of Lost Books." He lifted his gaze from the page to study her. "Hotho brought me a copy from Oldtown. He has a daughter he would have me wed." Lord Rodrik tapped the book with a long nail. "See here? Marwyn claims to have found three pages of Signs and Portents, visions written down by the maiden daughter of Aenar Targaryen before the Doom came to Valyria. Does Lanny know that you are here?" -AFFC, The Kraken's Daughter

• He possesses a working glass candle
• He doesn't exactly get along with the other maesters. In fact they have sent someone to Daenerys as well: "Others Seek Daenerys Too": The Citadel's Man

Marwyn's Route

Marwyn plans to take the Cinnamon Wind back to Slaver's Bay:

What will you do?" asked Alleras, the Sphinx.

"Get myself to Slaver's Bay, in Aemon's place. The swan ship that delivered Slayer should serve my needs well enough. The grey sheep will send their man on a galley, I don't doubt. With fair winds I should reach her first." Marwyn glanced at Sam again, and frowned. "You . . . you should stay and forge your chain. If I were you, I would do it quickly. A time will come when you'll be needed on the Wall." He turned to the pasty-faced novice. "Find Slayer a dry cell. He'll sleep here, and help you tend the ravens."

which should get him there quickly if they are able to resist Ironborn attacks:

He considered begging Kojja Mo and her father to take the wildling girl with them to the Summer Isles. That path had its perils too, however. When the Cinnamon Wind left Oldtown, she would need to cross the Redwyne Straits again, and this time she might not be so fortunate. What if the wind died, and the Summer Islanders found themselves becalmed? If the tales he'd heard were true, Gilly would be carried off for a thrall or salt wife, and the babe was like to be chucked into the sea as a nuisance. -AFFC, Samwell V

and leave soon:

"Two days, ten days, who can say? However long it takes to empty our holds and fill them again." Kojja grinned. "My father must visit the grey maesters as well. He has books to sell." -AFFC, Samwell V

but from the way GRRM has mentioned him in SSM's/foreshadowed him in the main series, I doubt his fate is to die in the Redwyne Straits.

If interested: All Aboard!: The Journey of the Cinnamon Wind

Marwyn in Slaver's Bay/Beyond

• Marwyn's Glass Candle

It is unknown if Marwyn took his working glass candle with him to Slaver's Bay or left it in Oldtown

• Quaithe's Warning

Unless Marwyn is the Perfumed Seneschal than Quaithe has warned Dany about everyone who was coming to Slaver's Bay at one point except for Marwyn:

"No. Hear me, Daenerys Targaryen. The glass candles are burning. Soon comes the pale mare, and after her the others. Kraken and dark flame, lion and griffin, the sun's son and the mummer's dragon. Trust none of them. Remember the Undying. Beware the perfumed seneschal." -ADWD, Daenerys II

also worth noting that Quaithe should know Marwyn exists since they both have glass candles (cue Quaithe = Marwyn theories).

• Marwyn's Role

If/when Marwyn and Dany meet, I think he will be doing the opposite of what most of the maesters want (magic gone) and will focus on getting her to embrace her destiny/magic. I think Marwyn takes one of the more practical approaches to "magic" (recognizing its power while also respecting how dangerous it is):

“Maester Aemon believed that Daenerys Targaryen was the fulfillment of a prophecy… her, not Stannis, nor Prince Rhaegar, nor the princeling whose head was dashed against the wall.”

“Born amidst salt and smoke, beneath a bleeding star. I know the prophecy.” Marwyn turned his head and spat a gob of red phlegm onto the floor. “Not that I would trust it. Gorghan of Old Ghis once wrote that a prophecy is like a treacherous woman. She takes your member in her mouth, and you moan with the pleasure of it and think, how sweet, how fine, how good this is… and then her teeth snap shut and your moans turn to screams. That is the nature of prophecy, said Gorghan. Prophecy will bite your prick off every time.” He chewed a bit. “Still…” (AFFC, Samwell V)

and:

What feeds the flame?” asked Sam.

“What feeds a dragon’s fire?” Marwyn seated himself upon a stool. “All Valyrian sorcery was rooted in blood or fire. The sorcerers of the Freehold could see across mountains, seas, and deserts with one of these glass candles. They could enter a man’s dreams and give him visions, and speak to one another half a world apart, seated before their candles. Do you think that might be useful, Slayer?” -AFFC, Samwell V

If interested: The Path Back to Westeros: Before Leaving Slaver's Bay

TLDR: Just a quick post on Marwyn the Mage's arrival in Slaver's Bay since it isn't discussed as often. GRRM has been building him up as a character since AGoT (in numerous plotlines) and so it will be interesting to see what effect he has on the plot (likely an advocate for embracing magic). Just one of the numerous subplots that must be untangled before Dany's return to Westeros.