So, some of this is taken directly from the book

with a few differences

She stood beside Ginny, horrified. There duels were so close to the school, there was no way the interior of the school would safe for much longer. She heard screams as a giant appeared, swinging a stone gargoyle.

"Let's hope he steps on some of them!" Ron's voice yelled nearby.

"As long as it's not any of our lot!" Tonks responded, as Ginny sent a well-aimed jinx at the group below them.

"Good girl!" A voice thundered, and Tonks spotted Aberforth rushing by. "They look like they might be breaching the north battlements, they've brought giants of their own."

"Have you seen Remus?" Tonks called after him.

"He was dueling Dolohov." shouted Aberforth. "Haven't seen him since!"

"Tonks." said Ginny. "Tonks, I'm sure he's okay—"

But Tonks was already running off after Aberforth, she had to get to Remus. She had to. She ran down a staircase, and found herself near the Entrance Hall. It was already littered with bodies, and curses were flying everywhere. She ducked curses, heading down a different corridor, surprised that she was already feeling the battle adrenaline that could keep her going for hours.

She slipped suddenly, landing on her side in a pool of blood. She pushed herself up, and then recoiled in horror. Arthur Weasley was laying there, his face and body mangled and bleeding, his glasses laying crooked on his face.

She let out a small scream. Arthur Weasley was dead, and she was drenched in his blood. She pushed herself to her feet, trembling. Her need to find Remus was growing stronger now.

She had to find him, he had to be alive.

She ran off, further down the corridors, sending curses here and there to help any students she saw in duels. They were children, they shouldn't be here. She heard a loud cackle, and there was only one person that that laugh could have belonged to. She spun, seeing Bellatrix cornering a very small blonde boy. There was a flash of green before she could hit Bellatrix was a curse and the boy crumpled.

"No!" Tonks screamed, running at her Aunt. Bellatrix turned, grinning madly.

"Ah, Nymphadora. I was hoping to see you." Bellatrix said, raising her wand at Tonks. "Crucio!"

Tonks dove to the side, the curse barely missing her. The wall beside her splintered at the force of the spell. She jumped to her feet again, sending a Killing Curse at Bellatrix, who had to dive as well to miss it. Her eyes widened.

"Oh, you are as dark as the rest of us." Bellatrix commented. "The good Auror, the perfect wife, the loving mother- but you're just as murderous as the rest of us."

"Only for you." Tonks sneered.

"It's interesting." Bellatrix commented casually, as the castle shook with another explosion. "Your darling mutt husband said the same when I killed him."

Tonks froze. No, she had to be lying. She was suddenly aware of the green light heading for her, and fell to the side, the spell exploding against the wall behind her, the debris raining down on her- and everything went black.

Remus fought, but he was tiring quickly. The only thing that kept him going was the idea of his wife, and son, both safe at home. But truly, the only thing that saved his life was when the battle ceased as Voldemort's voice rang out.

"You have fought-" said the high, cold voice. "-valiantly. Lord Voldemort knows how to value bravery. Yet you have sustained heavy losses. If you continue to resist me, you will all die, one by one. I do not wish this to happen. Every drop of magical blood spilled is a loss and a waste. Lord Voldemort is merciful. I command my forces to retreat immediately. You have one hour. Dispose of your dead with dignity. Treat your injured. I speak now, Harry Potter, directly to you. You have permitted your friends to die for you rather than face me yourself. I shall wait for one hour in the Forbidden Forest. If, at the end of that hour, you have not come to me, have not given yourself up, then battle recommences. This time, I shall enter the fray myself, Harry Potter, and I shall find you, and I shall punish every last man, woman, and child who has tried to conceal you from me. One hour."

Immediately, the Death Eaters vanished. Remus and Kingsley exchanged glances, and Kingsley nodded briefly. Kingsley sent a patronus to move around the castle, telling everyone to gather in the Great Hall- to bring the injured and the dead.

Remus, on the other hand, had produced a patronus but it didn't move. He had allowed his patronus to take its proper form, a wolf, but it just sat there instead of disappearing to travel to Tonks, as it was supposed to.

There was only two reasons a patronus wouldn't go and relay its message- and both meant that Tonks was here, at the battle.

I totally just made up the patronus thing at the end there, I have no clue if there's ever a time that a patronus won't go and spread its message but let's say there is. It makes sense, in my mind, that a patronus won't go relay its message if the person who is meant to receive it is dead or is physically unable to.