"Breakout" is a Marvel cross over event. A special prison called The Raft was built to house supervillians (seen in Captain America: Civil War). Somebody messes with the security systems and all the supervillain inmates are released.
"The Masters Of Evil" are a super villain team put together by Baron Zemo (badguy in Captain America: Civil War) to battle The Avengers.
"Under Siege" is one of The Avengers most iconic storylines. Baron Zemo puts together a team of supervillains "The Masters Of Evil" and then they wait until a time when everything is calm and The Avengers let down their guards, until different Avengers leave the base to do various different things, and then The Masters Of Evil attack the base and easily defeat the few Avengers inside... and easily pick off the other Avengers one at a time as they return to the base not knowing what is going on. Hercules get beaten to within an inch of his life. Captain America is captured and tortured. Jarvis (in comics past Jarvis was actually a person - see Agent Carter - who was an Alfred type butler for The Avengers) gets beaten into a coma and sent to the hospital (Jarvis was later used as a template for Tony Stark's AI program in Iron Man 1,2,3 Avengers 1 and in Avengers: Age Of Ultron Jarvis gets downloaded into the synthetic body gets zapped by Thor and the Infinity Stone and becomes Vision).