I know; I'm a bit idiosyncratic in my Dark is Rising slash, but I simply cannot make Will Gwion not my headcanon.
The words that Gwion is described as speaking in Welsh are given as Kristoffer Hughes' English translation, taken from his work From the Cauldron Born; the original Welsh is from the medieval Book of Taliesin.
'What I've always wondered is,' said Will, shoving his hands deep into the pockets of his jeans, and looking out over the rooftops of the city, 'do you know everything, all the time? Everything that happens, as it happens - and before?'
'Oh no,' said Gwion, 'no. That would be a burden, truly.' He crossed to stand beside Will, and leant against the rail of the balcony, his back to the buildings spread out golden below in the still, thick light. 'Rather, think of it as if I hold the keys to all the locks ever created in the world, or even dreamed of, to use as I will.' And in his brown callused hands there were indeed keys, a great ring full of them. Gold and brass and iron keys there were, some large and some small, some plain and others adorned with finely-wrought designs, even jewels. Turning them over and over so that they jangled with a quiet music, he said, 'In a way it is not so different from your own Gift of Gramarye, Will. Can you not look at a stretch of green ground, or the sky at night with its stars, and choose whether or not to name each growing thing or each singing sphere?'
Will nodded thoughtfully. 'That's true. I see.'
'Only, in my case, there are certain doors which cannot again be closed once they have been unlocked.' Gwion's hands stilled; for a moment his grey eyes were shadowed, and he said softly in Welsh, almost to himself, 'It is a river that flows; I know its might, I know how it ebbs and I know how it flows - '
'I know how many creatures there are under the sea,' Will finished with him in a low voice, and Gwion's bearded face broke slowly into the smile that creased deeper the lines of his face and stopped Will's heart.
'You know my words better than I do myself, Old One.'
Will rubbed his nose dubiously, and looked away. 'Well. I wouldn't say that.' Nor would he speak the rest of his thought: that how else was he to hold onto the hidden cornerstone of his reality, other than through words centuries old, half-remembered and less understood; that no matter through how many translations and transmutations and dreams Will pursued him, Gwion would still slip through the cracks of his life again and again.
The sun hung in the sky and the moon beside it, and the stars were faintly visible against the pale blueness of the firmament.
Gwion sighed, the smile fading from his face as if he guessed what Will had left unsaid, and he clasped Will's shoulder in a gentle squeeze. They were of a height; Will stockier, with the sandy hair that still fell into his eyes, and Gwion leaner, browner, grey-haired.
'You know you have long years yet in the world, Will, and your post of Watchman cannot yet be left. Bran needs you still.'
'Give me a time.' The fierceness in Will's own voice surprised him. 'Give me a date. If I only knew - ' He took one of Gwion's warm hands in both his own and on a sudden impulse pressed it to his lips.
Gwion sighed again, a sound as soft and longing as the far-off sea, and, closing his eyes, pressed his forehead against the younger man's. His free hand caressed the back of Will's head. For a long moment they stood there, each sharing in the other's exhaled breath.
Then, 'Take this,' Gwion said, and he placed something into Will's hand, closing the fingers over it. 'When you find the lock that this will open, then will you know it is time.'
And Will woke, alone, to find the cold grey light of a London winter filtering through the panes of the flat he shared with Bran. In his hand lay a small silver key - still cool to touch, not yet warmed by the heat of his body - its bow a small circle, quartered by a cross.