This is something I just thought of last night, I thought it would be kinda fun to see this happen. So here it is. Also, this based sometime during season 2 of Flash, after Barry's jump to SuperGirl's world.

"Let her go." Elsa demanded, hey blue eyes hardening like her ice, her hand stretched out toward the assassin. The assassin tightened his grip, his blade against Anna's throat tensed.

"I wouldn't do that, queeny." He smirked, his eyes twinkling. "If you kill me, you'll kill you sister as well. You aren't fast enough to kill me without me kill her." Elsa glittered her teeth, lowing her hand, her men fallowing and lowered their crossbows.

"What do you want?"

"Payment. Place your treasury on a small boat that you will give me, I will take Princess Anna here onboard with me till I'm out of the clove, where I will dump her. If you don't, I kill her."
"And you would fallow." Elsa glared, smirking. "Not a well thought out plan."

"I would, I don't the money, though I know you won't let me kill your sister and I would still walk away with all that gold... Really I just want to see you fall, sorcerer. Without it, you're standing with other kingdoms and your economy will fall and your kingdom… Crumble." The man grinned, Anna trembling in his hands. Elsa glanced at her before nodding harshly.

"Ok. Just… just let me get my servants get it." Elsa begged, stepping further away. The assassin grinned, nodding.

"Very well, your Highne-" He started, a sound of a distant cannon. "What's that…" He asked squinting at Elsa and her men, who stared past the open gates of Arendelle and past the village. The assassin tightened his grip on his dagger, glaring at Elsa. "What do you have up you're sleave Queen Elsa?"

"That not me-" Elsa started but her men shouted, causing her and the assassin to look out past her gates. A red and yellow lightning shoot over the forest covered terrain behinds Arendelle, weaving through the trees and shoot up the street and over the bridge. Elsa eyes widened, fear gripping her heart. Ice spread across courtyard as the lightning raced past the gates.

The lightning crashed into the assassin, the assassin and a man shooting past Elsa before sliding to a stop across the ice. The man in a red leather suit groaned, slipping slightly as he tried to tried to stand, Anna racing to Elsa who quickly hugged her, her soldiers raising their bows at the assassin and the man in red.

"Ouch… sorry. I slipped." He muttered as the assassin groaned, trying to stand as well. Elsa shot a blast of ice, encasing his lower body in ice. The man in red jumped, skidding slightly. Elsa frowned, directing ice at him.

The man eye's widened before he vanished in a streak of red lightning, wind blowing the two women's hair flying to the side.

"Where did he go?" Elsa's General shouted, stepping beside his queen as he and his men stared around nervously.

"Ok… I'm certainly not in Central City… or my Earth." Everyone twirled around, Elsa's arm holding Anna back behind her, ice and snow twirling around them. The man in red suit stood in front of them, staring at the fortress before grinning and pointing at it.

"Cool castle."

"Who are you?" Elsa demanded as a snowstorm started to build around them. The man, who had a white emblem with a yellow streak on his chest, stared up at the building snowstorm then back at her in awe.

"Are you a meta. No wait, medieval times… late medieval times, probably by those crossbows." He squinted at Elsa, before grinning again. "And if just guessing by your figure, you're the Queen."
"I'm Queen Elsa of Arendelle, the Snow Queen. Who are you!?" She glared at him.

The man studied her before chuckling. "I'm the Flash."