Eraklyon is Sky and Brandon's home world. Sky is the current king due to his father, King Erendor voluntarily abdicated the throne and gave it to him. Sky's mother Queen Samara is still the current queen until Bloom and Sky marry.
Eraklyon was mentioned several times in the first season but wasn't seen until Sky and Brandon took Bloom and Flora to meet his parents in Season 2. During the visit they learned that Diaspro was kidnapped and Sky immediately went to save her, raising Bloom's jealousy a bit.
In Season 3, Sky again invited Bloom and her friends to Eraklyon to celebrate Eraklyon's one thousand year anniversary. Just before Sky was going to make an announcement, Diaspro put him under Valtor's spell. And when Sky stepped forward to announce his love, he said that he loved Diaspro, and called the Winx, witches, and by saying that, he broke Bloom's heart (literally). Later, the Winx girls find Valtor's mark on Sky's shoulder. After the Winx escaped from Eraklyon (during which Stella earned her Enchantix) they went back to Alfea but went back some time later after Sky announced his engagement with Diaspro to remove Valtor's magic and break his spell. After they did, they fled before anything happened, during Palace 3D
which Diaspro was found out and banished from Eraklyon. Some time later Sky returned to Bloom (after she earned her Enchantix on Pyros) to personally and heartily apologized for what had happened.The Eraklyon Royal Palace