Madrigal Garden is a special location only accessible through use of the Music Ticket at the ferry landing in Ladoga Town. It is ultimately the location where the player is able to encounter and capture Meloetta.
When the player arrives in Madrigal Garden they will notice a number of people already on the island acting strangely and talking about a song that seems to have possessed them. Avoiding the NPCs, easily as they don't battle you, and traveling further into the garden leads the player to what appears to be the source of the corruption: a Meloetta. Upon interacting with the Meloetta the player learns something interesting: This Meloetta appears to be able to communicate with humans and also appears somewhat hostile.
The image seen to the right is what awaits the player in battle against this "evil" Meloetta. Aside from a difference in appearance, this Meloetta is also unique in that it is able to use the move Dark Sonata. This is the only time that the player will observe Meloetta in this forme, as it reverts to a normal Meloetta sprite after being captured by the player, though it retains the use of its signature move: Dark Sonata. Meloetta is encountered at level 100.
TIP: Make sure to save before fighting Meloetta, as it will not appear again if it is knocked out.
Once Meloetta has been captured or defeated, the garden's inhabitants will be freed from corruption and will regain their senses.