If Minerva McGonagall kept private her own inner demons, then she guarded even more fiercely her gods and her ghosts. So it came as a considerable surprise to almost everyone at Hogwarts when she announced her intention to take a sabbatical, of unspecified duration, following the end of the school year – and on the Isle of Iona, of all places. Not amongst the Muggles, of course, but with the druidic community that existed within and alongside the ancient Abbey – if one knew where to find it.

When Minerva passed the wards of the Castle to Filius on the night before she was due to leave, she saw the little man stagger for a moment as the magics settled into his bones; she knew the feeling, the coiled charms sleeping beneath her skin, the sense of a second awareness, inhuman but somehow living nonetheless, that echoed in her mind and gradually came to rule her pulse and her breath, her waking and her sleeping.

The wards themselves had come to Minerva after Dumbledore's death, but so riven and sick with shock had she been in that moment she had scarcely registered the weight of the Castle as it sank into her.

When Snape had returned as Headmaster, she had faced him boldly in the office that had been for so short a time her own. 'I suppose, Severus, that you expect me to pass the wards into your control?'

'There are many things I expect from you, Minerva,' he said, leaning back in his chair with his long white fingers steepled beneath his chin, 'and the first is that you address me from now on as Headmaster Snape. Is that clear?'

She looked down at him from her own considerable height, over the tops of her spectacles, just as she had when he was an insolent teenager lounging in Transfiguration class; how much has changed since then, she thought. But this was not worth a battle; there would be other times and places to stand on principle. With as great a disdain as she could muster, she said icily, 'As you wish, Headmaster Snape.'

A smile ghosted across his lips. 'Subservience always did become you so poorly, Minerva.'

She flushed, but replied evenly, 'And you have never been as skilled at being masterful as you like to think.' A flicker of a pause. 'Headmaster Snape.'

His face hardened. 'To return to the matter in hand,' he said coldly, 'I do not intend to take the Castle's wards onto myself. I am content to permit them to remain with you.'

Minerva was temporarily thrown. 'I beg your pardon?'

'Is it lack of hearing or of comprehension that plagues you?' Snape said sharply. 'I expect you to continue to hold the wards of Hogwarts Castle as you have done since Dumbledore's unfortunate demise. After all- ' slim pale wrists showed beneath exquisitely tailored cuffs as he stretched out both arms '—you must concede there is little to be feared by one in my position, as things now stand. However- ' and he lent in, intent now, black eyes holding hers '—you will swear to me to mention this matter to no one – not staff, not student, no one. Do you understand? I am giving you a token of my trust, Minerva, and if you should break it the consequences to you will be singularly unpleasant.'

'I understand.' But she did not, completely; not then.

Snape seemed hesitating on the verge of saying something more; but if so, he thought better of it, and dismissed her with a wave. Minerva had swept out, puzzled and angry, and more heartsick than she had been able to admit to herself or any other.

Over the following weeks and months she had kept her promise to remain silent about this strange arrangement between herself and Snape; in any case there were too few she could trust, and none of them needed an extra burden. But she watched Snape, and wondered; saw him handle the Carrows, seeming to endorse their reign of terror while subtly holding them back; saw his face grow ever whiter and thinner, which no one else seemed to notice for they feared him too much to look at him; and, half-guessing, heard Dumbledore's last words over and again in her mind - not as a plea for his life but as a command.

Finally she came to accept it; Snape had left her the guardian of the Castle so that she would better be able to protect the innocents who still lived there. And while this did not mitigate to any great extent the horror of day-to-day existence at Hogwarts, it gave her something which she clung to more tightly than she would ever confess to anyone: a thread of hope, that there was indeed a plan to end the Dark Lord's ascendancy, however convoluted and nightmare-like this plan might be. Thus even on the nights when she went to bed weeping – another thing she would confess to no one – each morning she was able to rise again dry-eyed.

Now, the sudden lightness that she felt at handing the Castle into the care of another left her almost giddy.

'A strange sensation, most strange indeed,' Filius was saying. 'Still, I suppose I shall become accustomed to it in time. I hope I shall be able to fill your boots adequately in your absence, Minerva, my dear – though not literally, of course.'

Minerva smiled and lowered herself into her comfortable armchair. 'Dear Filius, you will excel yourself as always. I have the utmost faith in you, of course; I would certainly not be leaving you as Acting Headmaster, otherwise!'

'And you cannot say when you will return?'

She shook her head. 'Forgive me, Filius, but I can't.' Her lips twisted wryly. 'Far be it from me to think I would ever say these words, but I need to heal, and to make some kind of peace with the past – and much though I love Hogwarts, I can remain here no longer for the present.'

Filius clasped her hands in his own. 'No one can ask more of you than you have already given,' he said warmly. 'You have been tested beyond any of us, since well before the Battle, and you have worked unceasingly since then. No other witch or wizard could have given us strength and hope as you have done these last years.'

Minerva, embarrassed, made a dismissive gesture, but Pomona broke in from her seat by the fire, where she had been silently watching. 'I heartily agree with everything Filius has said.' She waved her wand and the teapot danced around three tartan-patterned cups in a cloud of fragrant steam. 'However, since you clearly don't wish to discuss it at the moment, Filius and I will instead drink a toast to you – a toast of tea,' she added with a giggle; and Minerva was still so glad to hear the sound that even though the joke was, as always, appalling, she could not help smiling, although tears also pricked at her eyes.

Silently, solemnly, her two friends raised their cups to her; and Minerva bowed her head in wordless acceptance.

The next morning, early, before anyone but the house elves was awake, she took her small carpet-bag and walked through the still summer air to the edge of the grounds; the promise of heat hung in the cloudless sky but the grass was still wet with dew. Outside the great gates she paused for a moment, remembering Pomona's parting words the previous night. The short plump witch had wrapped Minerva in a warm embrace and then stretched up on tiptoes to kiss her on the cheek. 'Do your penance if you must, Min,' she had said softly, 'find your peace and then come home to us.' And with a smile and a look that showed she understood more about Minerva than the other witch had thought possible, she had trotted off to her own rooms.

Minerva took a deep breath and settled her hat more firmly on her head. 'Well, we shall see,' she muttered, and Disapparated without looking back.