Complex sentence: They were shown in a flashback in the episode "Hyperion" where the Squadron Supreme were stated to have been heroes on Hyperion's home world and were killed when the planet was destroyed in a civil war.
Split into simpler sentences: They were shown in a flashback in the episode "Hyperion". The Squadron Supreme were stated to have been heroes on Hyperion's home world and were killed when the planet was destroyed in a civil war.