Winter Soldiers

"Who are they?" "The most elite death squad. More kills than anyone in HYDRA history. And that was before the serum." "They all turn out like you?" "Worse."

- Steve Rogers, Bucky Barnes and Sam Wilson

The Winter Soldiers were HYDRA's elite group of assassins that were enhanced by a Super Soldier Serum similar to the one that created Captain America and the original Winter Soldier. They were all killed by Colonel Helmut Zemo.

History
"They speak thirty languages, can hide in plain sight. Infiltrate, assassinate, destabilize. They can take a whole country down in one night, and you'd never see them coming."

- Bucky Barnes

According to Bucky Barnes, the original Winter Soldier, the Winter Soldiers were an elite group of five assassins under HYDRA who possessed the highest kill count out of all the assets HYDRA possessed even before being enhanced by the Super Soldier Serum. They were injected with the Super Soldier Serum to replicate the Winter Soldier himself. Bucky was tasked to train the Winter Soldiers to see if they could overpower him, meaning the experiments were a success. To the delight of HYDRA, it was. However, the Winter Soldiers, except for Bucky, lost control and started a riot, resulting in all of them being put under deep sleep in Cryostasis Chambers.

They were found in 2016, by Sokovian Colonel Helmut Zemo, whom Captain America and Iron Man believed was looking for them to turn them into weapons for his own agenda. To the Avengers' surprise, Zemo had actually killed the sleeping Winter Soldiers by shooting them each in the head. The true reason why Zemo had located the HYDRA base where the Soldiers were kept was to find archived footage of Barnes killing Tony Stark's parents Howard and Maria Stark, in order to turn Stark against Rogers and Barnes.

Abilities
According to Bucky, the Winter Soldiers could speak thirty languages and blend in with any population without being detected and despite being a group of five, they could destabilize an entire country overnight. Even before they were enhanced by the Super Soldier Serum, they possessed the highest kill count of all of HYDRA history.