(A/N) While I was on a train ride I thought to myself, "I wonder what shadowhunters do in the winter"

Well, here's my answer:

Everyone is slightly OOC, because really, who would take a snowball fight this seriously? Also I used more modern dialogue.

The Battle of the Ages

When Jem had first told her about it Tessa had almost laughed out loud. It seemed so childish, so silly. But Jem explained to her it was a tradition at the London Institute. Tessa decided to chalk this one up to the Nephilim's strange traditions.

Every year the shadowhunters at the London Institute would form teams (usually Will and Jem against Henry, Charlotte and Jessamine) and engage in a huge snowball fight. As Tessa understood it this battle was much more elaborate than your average backyard snowball skirmish. They had been preparing for two weeks, developing strategies, and designing their fortresses.

Will was gone at breakfast the day of the fight, putting the finishing touches on his "snow wall defense system." It was there Jem asked Tessa to join their team. At first Tessa was shocked by the notion. The snowball wars seemed so serious and hard-core, but Jem insisted it was all in good fun. It was hard for Tessa to resist, she hadn't had a good snowball fight since she was a child, back in New York. And since Jessamine was participating, she couldn't call Tessa out for being "unladylike."

So that afternoon she borrowed a pair of trousers from Charlotte, put on a pair of mit

tens and tucked her hair up in a warm cap in preparation for the battle. When she and Jem stepped outside Tessa was caught again between bursting out laughing and just standing there in awe. The "battlefield" was in front of the Institute and more spectacular than anything Tessa had ever seen before. The two forts were about 200 feet away from each other. Jem starts walking toward one, Tessa presumed it was theirs. This fort was a 5 foot high wall of snow that was about 25 feet long, and curved around to create an oval shape. In front of it was a network of trenches, about 5 feet deep. In front of the trenches were two smaller forts on either side. They were four foot tall walls in an eight foot circle. They were stocked with snowballs and large dangerous looking ice chunks. Jem led her to the trenches and helped her down into one. He jumped down himself and then led her around the maze of trenches. Jem started explaining as they walked,

"The only way inside the fort is through the trenches. I designed them last June, so Will and I know our way around them. Be careful going around the trenches, some are rigged to collapse and trap you in a ditch." Jem suddenly stopped, took out his stele and carved an "open" rune in wall. The snow melted away to reveal an underground tunnel with witchlights embedded in the walls to show the way. Tessa looked back to see the hole Jem had created was freezing back over, once again looking like a wall of snow.

The tunnel led to a set of stairs that went up into a dome shaped room. The room was about 15 feet in diameter, with a large plateau of ice in the middle that served as a table. On it was a map of the battlefield, with notes and arrows scrawled on it. Will was leaning over the map writing something on it.

"Where are we?" Tessa asked.

"An igloo disguised as a pile of snow," Jem answered. "Oi! Will! Tessa's joining our team." Will turned around with a raised eyebrow.

"Really?" Will asked with a grin, "You can be my personal cheerleader!" Tessa rolled her eyes, choosing to ignore him. Will continued still grinning, "Are you up for it? We have different rules here. There is no going inside to warm up your hands and drink some hot cocoa."

"I'm up for it," Tessa said with a shrug, "And I hate chocolate, remember?" Tessa walked over to the map and traced the different colored lines with her fingers. "What do these lines mean?" she asked looking up at Jem.

"Will was mapping out our first attacks. We were going to use the Weeping Marmoset attack, but we don't have enough resources or the right terrain for that. So we're going to use the Wombat of Doom tactic." Jem said.

"Am I supposed to know what any of this means?" Tessa asked, gesturing to the lines and notes on the map.

"Nah," Will said with a wave of his hand, "We'll just tell you what to do" So Will explained to her the plan. It was fairly simple, despite its elaborate name. She and Will would go around the back of the Institute to sneak into their opponent's fort from behind while Jem would play defense and bombard anyone who came near their fort with snowballs.

They set their plan into action the second Sophie rang the bell to signal the start of the game. They ran at top speed, neither of them tiring out, and they arrived at the other side of the Institute in a couple minutes. They stopped to peek around the corner to see if there was anyone guarding the back. There was. Jessamine was there sitting on a pile of snow, looking bored.

"Yes!" Will whispered with an evil grin "I call dibs on hitting her in the face with a snowball!"

"No!" Tessa hissed back "I get to"

"No way! I called dibs" Will turned around to face Tessa. Tessa crossed her arms and replied,

"Too bad I get to"

"That's not fair! I've been waiting for this opportunity since I first met her eight years ago!"

"I deserve it more! I bet she didn't make you go shopping with her for hours and drone on to you about fashion and gossip and set you up on dates with tons of men that she picked out!"

"Well I -Wait- you went on dates? With guys? Who?" Will said, his protective instincts kicking in.

"Gabriel Lightwood, if you must know, but that's not the point! The point is that Jessamine is the most annoying person in the world and I, not you will hit her in the face with a snowball," Tessa said

"You went on a date with Gabriel Lightwood?" Will asked, still hung up on her earlier statement.

"shh!" Tessa shushed him. Their argument had gotten louder and Jessamine was finally taking notice.

"Hello? Charlotte?" Jessamine called, looking around. Tessa looked back at Will with a wicked grin. She already had two snowballs in her hands. In one fluid movement she sprung out from behind the wall and chucked the first snowball at Jessamine. She purposefully missed and hit her in the stomach to get Jessamine to look at her so she could get a clear shot at her face. She flung the second snowball and it hit Jessamine right in the middle of her face. The snow went into Jessamine's eyes and mouth. Jessamine sunk to her knees spitting out snow and wiping it from her eyes.

Will stepped out from behind the wall.

"Nice shot." He said, impressed by Tessa's throwing skills.

"Thanks." Tessa said with a grin. "I thought it would be enjoyable… It was."

"Tessa? How could you? I thought you were my friend!" Jessamine half sobbed.

"Oh, but I am Jessie, dearest!" Tessa said, making Will shake in silent laughter.

"I quit!" Jessamine shrieked with a scowl, " I'm cold and wet and I ruined my new scarf."

"She didn't last very long." Tessa commented.

"Longer than last year." Will replied with a shrug. At that remark Jessamine got up and left in a huff, only to trip and land in a deep snow bank. Tessa tried to suppress her laughter, but eventually joined Will laughing so hard they had to lean on each other to stay upright. Once their laughter subsided they got to work trying to find the entrance to their enemy's fortress.

They started by circling the outer wall. After two laps they determined that 1)there was no visible entrance and 2)Charlotte and Henry were occupied elsewhere, but they figured Jem could handle them. So they decided to adopt Will's approach: destroy everything. Will took the medium sized shovel off his back and punched a hole in the snow wall big enough for Tessa to crawl through. Tessa wriggled through and stood up on the other side and shook the snow out of her collar. Then she crouched down and looked through the hole she just crawled through.

"Hurry up!" She called "I thought you were supposed to be fast!"

"In every meaning of the word." Will said with a chuckle. Tessa rolled her eyes and muttered something under her breath. Will stood up on the other side of the wall and surveyed the area. All it seemed to be was an area of snow. No igloo no stairs going under. They stood there for a moment, puzzled.

"Do you think it's a decoy?" asked Tessa said walking forward a few paces.

"No," Will said slowly, "This is where it is supposed to be…" he crossed his arms and furrowed his brow, deep in thought. "Don't walk out any further, it might be a trap. We should test the ground somehow."

"Well, there is one way." Tessa said looking up at Will.

"What-"Will didn't finish his sentence because Tessa had thrown all her weight against him, pushing him into the middle of the snow field. Will staggered, but didn't fall.

"Yep, looks pretty stable." Tessa said with a smug grin. Will just laughed and shook his head. Looking around he took a small step back. That one half of a step caused the snow in a twenty foot circle to crumble and give way to a huge hole. Will wobbled for a bit, desperately trying not to fall, but eventually tumbled down into the pit. Tessa dashed to the side of the hole with a worried expression on her face.

"Will? Will are you alright?"

"Just peachy," Will replied with a groan.

"I can't believe you of all people fell in the hole, "Tessa said with a laugh.

"Believe me, neither can I," Will grumbled. Tessa continued laughing, much to Will's irritation. Tessa quietly picked up a handful of snow and tossed it into Will's collar.

"Hey!" Will said, looking up at Tessa, "What was that for?" he demanded, brushing the snow out of his collar. Tessa just started laughing harder, but finally stopped long enough to say

"Because I can!" Then she started giggling again, earning her a steely glare from Will.

"Could you be any more annoying right now?" Will said, throwing his hands up in the air.

"I don't know… Mr. Herondale." Tessa started laughing yet again, and Will couldn't help but join her.

"Just help me get out of here," Will said.

"Okay, how are we going to get you out of there?" said Tessa. She couldn't see how even Will, with his mad shadowhunter skills, could get out of the ditch. It was at least twenty feet deep, with soft snow at the bottom, the kind that took you twenty minutes to walk ten feet. It seemed that Will also had no brilliant plans for getting him out of this. "Sorry, Will! I guess I'll have to just leave you here." Tessa said with mock sorrow. She scrambled up from her crouching position, and ran toward the other wall, careful not to fall in the hole.

"TESSA!" Will shouted after her "WAIT! WHERE ARE YOU GOING?" He tried in vain to struggle out of the waist deep snow. Tessa just laughed and started climbing over the far wall of the field.

She jumped down to the other side and stood up. Suddenly she realized that the snow around her was glowing green. There was a loud bang that sent Tessa flying. She landed painfully on her back next to a big pine tree. After a few seconds she opened her eyes and could barely believe what she saw. All the snow had completely melted in front of the institute. All that remained was large puddles of water scattered around. Since all the snow was gone the field was flat so she saw everyone else slowly recover from the blast and stand up. They all looked around in bewilderment until all their eyes fell on a particularly sheepish looking Henry holding a small box with buttons on it.

"It wasn't supposed to melt the snow…" Henry muttered, scratching his head. After a second of dead silence everyone erupted in laughter and didn't stop even when they were gasping for breath and clutching their sides. Everyone except Will, that is, because he was still in the ditch that was now half full of muddy water. Tessa realized this when she was in the middle of laughing uncontrollably. Gasping and out of breath she managed to get out a few sentences.

"Jem! Can you come help me torment Will? He's stuck in a ditch."

"It would be my pleasure," Jem said with an evil grin.

They walked over to the ditch, shushing each other's snickers. Tessa kneeled by the mouth of the hole with Jem.

"What the devil happened?" Will shouted, looking very, very confused. Tessa laughed,

"I'll give you a clue: it involved Henry and a strange invention."

"You have got to be kidding me." Will muttered. After a minute he said, "So are you going to get me out of here?"

"I don't know…" Jem said crossing his arms and shaking his head, "Tessa, what are your thoughts?"

"Well, we are in a position of power here, Jem" Tessa responded in the same contemplating tone as Jem. "If Will really wanted to get out of that hole we could, oh, I don't know make him do something humiliating…" Tessa finished with an evil grin.

"Whoa!" Will said, with a terrified look in his eyes, "When did you get so evil?"