Dark Flare is a type (specifically a counterpart of the Special Bosses) of boss that emits dark aura and is stronger than their original counterpart. They also have a better moveset that can hit your units hard. When defeated, they always have a fixed drop of 5 Gold Boxes. The following stages that can be seen with these bosses are:
Appear as an uncommon invade in the following Special Boss stages from Volcano Tortoise to Three-Headed Serpent (exclude Castellated Dragon and Evil Gargoyle). Although rates are low, they go a little higher as rank goes to 5 or 10.
Dark Flare Rush!!
A 4 Battle stage against Dark Flare Tortoise to GEKIO. They are lower in level from the Special Bosses and still drop 5 Gold Boxes. Often a guerilla stage.
Dark Flares drop a 4 star Upgrade stone that has an accessory ambered to it. These items are worth 100pts for upgrading and sell for 10,000,000 coins! Though there are 7 kinds, they are one of the same.