A/N: This is based on a bookverse plot bunny from Birch Tree:

How did Pippin and Gandalf knew that Merry had been riding with the Rohirrim? In the book, it is clear that Gandalf and Pippin were already waiting for Merry coming along with the survivors, and then Gandalf sends Pippin to look for him and we all know all the rest. But how did he know? They supposedly thought him safe at Edoras with Eowyn, so who told them he actually had disobeyed Theoden’s order? Eowyn in a conscious moment? ... Or is it also this the "something grievous" Gandalf saw on the battlefield from the citadel?

Lines in italics taken from "The Pyre of Denethor", ROTK

***

Warring Duties

When the horns sounded across the fields of the Pelennor, piercing the dissonance of battle with a note so bright and true that Pippin nearly wept to hear it, Gandalf’s countenance only darkened when the valiant call of the Rohirrim met his ears, signalling the turn of the tide.

“Evil and sorrow will come of this,” he had muttered when Pippin frantically told him of the Lord Steward’s madness, and Pippin’s heart quailed as he wondered what could possibly be worse. From the few unhappy choices he possessed, he had done the only thing he could think of when he pleaded with Gandalf and Beregond to stop the evil darkening the very heart of the City. They were wiser and stronger, each of them, and he was just a hobbit, sworn to the Steward’s service for so brief a time.

Yet he had forced Gandalf to make a dreadful choice in his turn, and Pippin’s joy at hearing the sound of hope singing from the battlefield diminished quickly as Shadowfax bore them back up the ruined streets toward the citadel, and he bowed his head in dread, sensing from the wizard’s mood that it was too early to hope for anything, whether it be Faramir’s survival or their own. Gandalf was troubled and preoccupied by what was happening out there on the battlefield, a different task – a different fate – from which he had been unwilling to turn aside even to help the Steward’s son, and Pippin grew more and more uneasy about what dark foresight was weighing on Gandalf’s heart.

***

“Now I must go down to meet those who come. I have seen a sight upon the field that is very grievous to my heart, and greater sorrow may yet come to pass. Come with me, Pippin!”

As the wizard continued to give his instructions to Beregond, Pippin stood quietly beside his friends, still crushed by horror. When Beregond, grey-faced, turned at last toward the Citadel, he glanced at Pippin one more time with an unfathomable look of sorrow in his eyes, and Pippin’s heart grieved for his friend as he watched him walk away. If only he were stronger, and had stopped Denethor himself! But there was no time now to wish such things; what was done was done.

By the time Pippin’s attention turned back to him, Gandalf was already walking away down the street. Heavy rain began to fall, sending up clouds of steam and smoke from the fires blazing all around them, but Pippin hardly noticed it in his determination to reach the wizard. He ran to catch him up, and put a trembling hand on his arm. “What is it you’ve been talking about all this time?” he asked. “I'm frightened, Gandalf. You said others would die... what have you seen on the battlefield? Who were you meant to be helping?”

“We are going to meet them now, my lad,” Gandalf replied, walking quickly. “They are being brought into the City.”

“Who? Gandalf?” Pippin persisted, struggling to keep up as he distractedly wiped the rain from his face with one hand, leaving dirty streaks across his cheeks.

Gandalf looked down at him, and his expression softened, though the sadness in his eyes lingered. “I saw the Witch-king lying in his ruin on the battlefield, and beside it two that should not have been here. They must have ridden without the King’s knowledge, for he would have sent them back had he known they were riding to war with him, yet –”

Realising that Gandalf was struggling to tell him something terrible, Pippin let out a frustrated sigh, the tightness in his chest coming not from the effort to keep up with the wizard’s long stride but from fear, and he stopped, angry and afraid that he might cry.

“Merry,” he said sickly, “it’s Merry, isn’t it? He’s out there.”

“Yes, my lad,” Gandalf sighed. He stopped and knelt down before Pippin, clasping his shoulder firmly. “All may not be as it seems, and I am not yet sure what I saw from the walls. But I know that Merry, at least, will walk into the City." He looked deeply into Pippin's frightened eyes and smiled, then stood up and turned to continue down the street, gesturing to Pippin to do the same. “We must make haste.”

As the last bit of news sank in, Pippin stood still for a moment longer, watching after Gandalf in amazement. He'd known really, deep in his heart, that Merry would come to Minas Tirith, but he wasn't prepared for the thought of seeing him again so soon, especially not when he had feared the very worst. His heart lifted, joy and gratitude overwhelming him, and for a moment he forgot that the wizard had ever had anything but good to tell him as he ran once more to catch up. “Come on then, Gandalf!” he exclaimed, quickening his pace determinedly until he found himself running ahead of him.

They passed heaps of rubble and bodies lying crushed beneath them, and flames danced around them still, despite the rain. Pippin tilted his head back to feel the raindrops cool his face from the fierce heat, trying not to look too hard at what he saw around him. The devastation made his heart ache, and he tried instead to think of nothing more than finding Merry again around the next corner.

Even as he did so, the wizard’s words came back to him and he ran through them in his mind, frowning. “Gandalf?” he called, looking over his shoulder anxiously, “I know you’re not sad that Merry is here and on his feet. Who else was there on the battlefield? And what do you suppose Merry was doing near the Witch-king?”

“I believe your cousin may have had a hand in slaying him,” Gandalf said frankly, his lips curving into a small smile as he considered the startled hobbit.

Pippin was lost for words. "Good old Merry!" he exclaimed softly, when he recovered from his surprise. He smiled to himself, feeling a rush of pride for his cousin and no small amount of vindication on Frodo's behalf; but when he looked up again, he saw that the wizard’s smile had faded.

“I fear that King Théoden and his niece have paid the price for my absence,” Gandalf admitted as Pippin met his gaze.

Pippin heaved a deep sigh, looking away sadly as he remembered the old king and how kindly he’d thought him. “I’m sorry, Gandalf,” he said. “If I hadn’t found you, you could have stopped it. That’s what you wanted to do, isn’t it? But Faramir…”

Gandalf shook his head impatiently and gave the young hobbit a stern look; “You were Faramir’s only hope. The Enemy was working to divide us, and you had no other choice but to bring me to his aid. It was rightly done, and courageous, and I do not wish any apologies!”

“Yes, Gandalf,” Pippin murmured, and hurried along quietly for a while. “But what about the king’s niece?” he asked suddenly, his voice wavering. “Do they send women to war in Rohan, Gandalf? No,” he remembered. “The king didn’t know, did he? How dreadfully sad.”

“She was young and brave,” Gandalf said quietly, “and she would not have wished to be left behind while the men went to war.” Pippin looked up with a curious frown on his face and a question on his lips, and Gandalf paused and smiled down at him fondly. “I have known her for many years too, my lad. I suspect that Merry found in her a kindred spirit, and that is why they are both here.”

His expression darkened again and he sighed, “Even she would not have expected to strike such a blow against the enemy, and nor would your Merry, of course; but as you well know, my dear Pippin, great courage may be kindled when those we love are in need.” He stopped, and his brows knitted together in a deep frown. “This deed was foreseen, of course,” he muttered to himself darkly as Pippin strained his ears to hear the mysterious words. “A strange fortune indeed that he met such unlikely foes.”

“Is that good, then, Gandalf?” Pippin asked. “I don’t understand. I thought only you could kill the Witch-king.”

“Such has proved to be our advantage,” Gandalf replied cryptically. “It seems the Enemy may only be felled by those He least expects.”

Pippin raised his eyebrows and smiled gamely at Gandalf. It seemed as though Merry had got himself caught up in all sorts of deep and mysterious things, and now he was all the more eager to find him and make sure his Merry was still the same hobbit.

It was a long walk down to the lower levels, and dusk was falling across the City. By the time they came there the fires had died down beneath the rain, and foul, black water ran in rivulets down the road. Eventually, Pippin pointed eagerly as he saw the glimmer of torchlight shining on the walls ahead of them, and then men appeared, carrying a litter covered with a golden cloth, and behind it came another. Around them, people bowed down and took off their hats in silence as the procession passed through the streets.

Pippin looked frantically for a small figure going with them, but he couldn’t see one and, forgetting all courtesy in his haste, he ran up to the men bearing the first litter. “Is Merry with you?” he asked them. “Or, rather… Meriadoc, of the Shire. He fought beside the Lady Eowyn, I think, and he was supposed to accompany you into the City. Do you know where he is?”

One of the men started when he laid eyes upon the hobbit and Pippin's eyes locked with his, his heart lifting to see a spark of recognition and curiosity soften the lines of deep sorrow etched into the man's battle-scarred face. The other soldiers glanced about them for their erstwhile companion, and murmured in surprise to find him gone.

“Master Meriadoc was with us,” the man said, and looked behind him into the gathering gloom. “He must have lost his way.”

How could he have lost his way? Pippin followed the soldier's gaze and glanced back up at him worriedly, but the man simply inclined his head solemnly, unable to say more, and the party continued its march up the street.

Pippin bowed his head as the litters passed, and when they had gone, Gandalf came to him hurriedly. “The Lady Eowyn lives yet! I am going now to the Houses of Healing. You must find Merry, my lad, and bring him there without delay.”

Pippin nodded anxiously, needing no encouragement, and there they parted ways. For a long time then, as Gandalf went with the stricken Lady of Rohan to the Houses of Healing, Pippin wandered the winding streets of the City, searching for a small hobbit, lost in the dark.