Head Cannon:
When Ron Saved Harry
Severus Snape's long black cloak silently slithered along the soggy and dead leaves of the forest.
With only his wandlight as company and the sword of Gryffindor's empty coldness in his Slytherin hand he was entirely alone but for the slowly growing cold words of Albus Dumbledore instructing him to leave Harry the sword of Gryffindor, under the conditions that he would have to commit an act of courage to retrieve it.
As much as Snape disliked Dumbledore's cryptic warnings, and even in death the fact he could not wholly trust him, he had no choice but to obey. And besides, such a brilliant idea such as the one he had hatched in the office whilst talking to the portrait of the old headmaster deserved its chance to take place.
Walking around, he could not see Potter, or Hermione Granger, yet knew they were here from the information passed on to him by Phineas Nigellus.
He had to admit, Miss Granger's expedient act of placing the portrait of the former Slytherin headmaster in her handbag had been most fortunate in locating them without showing himself.
Standing still, he secreted himself behind a tree before closing his eyes...
Summoning her image...
Her smile...
Her smell...
Her eyes.
A Patronus sprang forth from his wand and cantered around in a circle before walking slowly back towards him. Her beautiful shining eyes looking at him in wonder, quizzically.
Smiling very faintly, he flicked his wand at the sword of Gryffindor as it disappeared.
The doe looked at where the sword had been, then looked back to Snape.
"...The sword has been secreted beneath a frozen pond over there...I want you to go to it...He will follow you to it..."
The doe bowed her head and began to stroll away casually, not leaving a single mark in the snow, nor a sound from her hoofs.
Snape waited with baited breath, watching his own breath cast its own weak, pathetic interpretation of a Patronus against the cold night air before watching it fade quickly, embraced by the night.
The doe was walking slowly and steadily to the place he had selected.
"...show yourself, Potter." he enthused, his black eyes near strained with effort, urging the boy to do as he asked.
After an eternity, from nowhere...he appeared.
So much like his father...
The dishevelled hair. The glasses. The stance. The strut...
Yet, this swagger looked...tentative, the wand he held lit in front of him as he gazed at the doe who was leading him towards his destiny.
He walked slowly, too slowly.
"...Hurry..." Snape urged, looking around for Snatchers, for death eaters, for anyone who shouldn't see him, or Potter.
Finally, the doe walked onto the ice of the pond, and stood as she transformed into a ball of brilliant energy, slowly sinking like a frosty sun into the ice, leaving no mark.
Potter looked confused, looking deep into the misted ice and unwittingly seeing what Snape had placed there.
Snape felt a wave of relief as he saw Potter cast a spell to break the ice and began shedding his clothing.
Watching carefully to ensure he got the sword from the pond he began to hope that Potter had learned how to swim at some point in his muggle life with her sister until he was distracted by a flash of metal on Harry's chest.
The locket?
The horcrux...He had found it...
Harry jumped into the pond and was submerged momentarily before he bobbed back up, gasping for air, steam rising from his body, his face, his breath coming out in raspy chokes.
Snape watched, his hand wrapped around his wand, the other hand prised on the tree trunk, watching for what seemed eternity.
A silent, relentless guardian for an unwilling ward.
He was submerged again, searching for the sword...
Surely he would find it...
It was a huge metal sword, its hilt covered in rubies, at least a foot and a half in length.
Bubbles appeared on the surface of the pond...a crunching noise muffled by water.
…
He had been under for too long...
"...Come on Potter..."
…
More bubbles
…
He would have to rescue him...
Snape staggered forwards, preparing to run and save Harry, silently planning how he would wipe his memory as soon as he had done it, find Miss. Granger and wipe hers, rescue Potter from his own stupidity when he saw another running towards the pond.
Not a snatcher...
Not a death eater...
A red haired man who reached down into the pond, pulling Potter to safety.
Potter gave a gasp and a splutter, coughing up water and wheezing before reaching for his glasses.
...Ronald Weasley.
Potter's best friend.
Snape stood back, breathing in relief before retreating back to the trees.
He had done his part...the next was theirs...and theirs alone.
Grasping his wand, he disapparated into the darkness, hoping that when he saw Potter again, he would look into his eyes and explain all of this, in some way, that whatever he did, he did for a love that death could not end.