The Sky Citadel is the eighth area of Sonic Boom: Rise of Lyric. It is a colossal and marvelous city from the Ancients, located high up in the skies above Bygone Island, held suspended amongst the clouds through mysterious technology. Virtually untouched by time, it is an archaeological marvel.
In the quest for the final Chaos Crystal, Team Sonic journeyed to the Sky Citadel to find the Sky Crystal. However, they were tricked by Lyric to relinquish their Crystals to him in exchange for Sonic's life. Not beaten though, Team Sonic pursued Lyric through the floating city to stop his plans to destroy the world.
A thousand years ago, the Ancients hid away the Sky Crystal, one of their own Chaos Crystals, away in the Sky Citadel so that Lyric would not be able to find it.
When Team Sonic tried to reach the Sky Citadel through a teleporter to get the Sky Crystal, only Knuckles and Amy got through it as it was only designed to take two. After journeying through the city, Sonic and Tails managed to reach them in the Tornado. Soon after, the team found the Sky Crystal, but as they claimed it, Destruction Troops surrounded them. Lyric appeared and explained that he let them find the Crystals so he could then take them from them. Sonic refused to hand them over, but Lyric threatened Tails, Knuckles, and Amy that if they did not, Sonic would die. Not wanting to risk Sonic's life, they relinquished the Crystals. As Amy demanded Lyric to keep his promise however, Lyric revealed that he lied and had his robots shoot Sonic down in an large blast.
As Lyric left with the Crystals, Tails, Knuckles, and Amy dug through the rubble, fearing the worst. To their relief, Sonic had survived, and the group took off to stop Lyric. With the Sky Citadel collapsing around them from Lyric's robot army, Team Sonic reached their only means of escaping: the Tornado. Though a chuck of rock broke the road as they got ready to leave, causing the Tornado to fall, Tails got the plane flying and team began heading towards Lyric's Lair to stop the villain once and for all.