The Mistro

The mistro is a secret boss that you can fight by using the Call of the void skill on the gateway at the very beginning.

When you use the skill the gateway will activate, and open a portal. IT IS HIGHLY RECOMMENDED YOU 100% THE GAME FIRST AND GET A LOT OF HEALING/ANTI CORROSION/MANA REGEN SPELLS.YOU WILL NEED THEM.

When you enter you head to a place called The Mirror. In this place you will find a world that has been fully corrupted (aka. the bad end) and fight heavly buffed enemies. you can't fight the tuba king or queen because the version of your Mistro killed them instead of sparing them like you did in the good ending/ true ending.

Beware while you are in this 'Mirror' you will continuously take corrosion/true Damage. you can negate this using a purging song, also you will get another debuff on all of your allies. This debuff is called Whispering.It will give you a +10% mana combsumption on all of your spells.When you eventually get near the Mistro's domain, A line of your Mistro's dialogue will show up saying "You feel a chill up your spine. Continue?" If you say yes, then get ready for The most agonizing fight of this game.