The Triskelion is a large building complex, that serves as the main headquarters for S.H.I.E.L.D.. It was destroyed, being smashed by the Helicarriers when Captain America and his team stopped Project Insight.
Marvel Cinematic Universe
In 1989, Hank Pym went to Triskelion and submitted his resignation to Howard Stark, Mitchell Carson and Peggy Carter after learning that SHIELD wanted to replicate its formula. Pym hit Carson against the table after he insulted him; Carter and Stark unsuccessful tried to convince him but Pym already had taken its decision. When Pym left the room, Stark told Carson that he deserved the beating.
After a successful mission, Steve Rogers went to the Triskelion to confronted Nick Fury about the motives behind Natasha's task on The Lemurian Star. Rogers told Fury that he was tired of his secrets. To prove otherwise, Fury took Rogers the place where the Project Insight was under preparation. Fury told Rogers the reasons of the project; however Rogers was very concerned that Project Insight simply threatened people in the name of protection. As Fury told him SSR also made mistakes, Rogers replied that were mistakes in the name of freedom. When Fury can not access the information that Romanoff got him, he personally went to request to his old friend, Alexander Pierce to delay the launch. Pierce agreed only on condition that Iron Man go to the party of his niece.
After the supposed death of Nick Fury, Steve Rogers was summoned by Alexander Pierce to talk. During the talk, Pierce told Rogers that Batroc was hired by Fury to kidnap the Lemurian Star. Pierce also asked Rogers the reasons that Fury was in his apartment, Rogers replied only to tell him not to trust anyone. At the end of the talk, Pierce warned Rogers that he would find the murderer of his friend and no one would cross on his way. Immediately, Pierce sent the Strike team to arrest Captain America. However, the First Avenger managed to escape. When the hunt began, Sharon Carter asked Jasper Sitwell why they wanted to arrest Captain America, Pierce replied that Rogers was hidding information about the death of Director Fury.
Days later, the World Security Council met at the Triskelion to witness the launch of Project Insight. While Pierce and the other counselors expected the launch, Captain America gave a speech in which he revealed the infiltration of HYDRA within S.H.I.E.L.D.. This revelation led to a battle between HYDRA agents and loyal agents of S.H.I.E.L.D., although the technician Cameron Klein refused to launch the Helicarriers, but the double-agent Brock Rumlow was able to send it into the orbit. Meanwhile, Alexander Pierce took the other council members hostage and explained that really was the Project Insight. As Singh was against Pierce's ideals, he threatened to kill him; but it was disarmed by Black Widow disguised as Councilwoman Hawley. Shortly after, Nick Fury comes to help Romanoff to reveal all the secrets of HYDRA and SHIELD. Using a special device Pierce killed the others members of the Council and also threatened Romanoff to do the same with her, after see how the Insight Helicarriers were destroyed each other. When the battle was ending, a Helicarrier collapse against Triskelion, destroying it completely.
When Thadduess Ross visited the New Avengers Facility to present the Sokovia Accords to The Avengers he showed footage video of The Avengers collateral damage during their operations, one of them showed during the Battle at the Triskelion of the S.H.I.E.L.D. Helicarriers being destroyed and falling near the Triskelion as people flee for their lives.