The Arcane Dragon. This Mage Boss appears in Boss Monster: The Next Level.
Based on the text of Nicolius's Level Up Ability, the ability seems to be rather situational, and seems to work best in a game where there are more than two players. This Boss' Level Up Ability, however, is aimed at getting the player to catch up if they are behind, and works well with cards that allow the player to keep Spell Cards in the hand, like Liger's Den.
It is unknown whether, in a game that has more than two players, if the Ability applies if each opponent were to get a Soul that turn, but the player with Nicolius did not.
Formerly known as TBD, The Blue Dragon. The Brothers auctioned the naming rights on eBay, in a manner similar to the original Boss Monster Kickstarter, when a backer could buy the rights to name the Golden Dragon, now named Kirax.
All proceeds made for the naming rights have gone to charity.
The name of this dragon is similar to "Nicol Bolas," a planeswalker Elder Dragon from the game Magic: The Gathering. In the Magic: The Gathering storyline, Nicol Bolas is a powerful magic user and the oldest living entity in the ingame universe.
The name of the dragon is also similar to Nicolus, an indie video game company. It is unknown whether this is a coincidence, or if the company themselves bought the naming rights to this Boss.