The Contemelia are an alien race of neutral, multiverse-travelling 'observers,' and the original masters of the Anihilaarg.
The Contemelia are fifth-dimensional beings whose true forms cannot be perceived by three-dimensional beings. Most three-dimensional beings would instead see a Contemelia as an animate projection of what holds the most emotional sway over them (a living version of the Mr. Smoothy mascot in Ben Tennyson's case, Rook Da in Rook Blonko's case, a duplicate of himself in Maltruant's case), while a genetometaphomorphic lifeform such as Skurd would instead just see a Contemelia as a mass of energy.
The Contemelia are known among the universe as stories. According to Rook in So Long, and Thanks for All the Smoothies, the stories state that the Contemelia would travel across the multiverse and play practical jokes on the inhabitants, but if they did not like a particular universe, then they would use the Anihilaarg to educe it to nothingness. In A New Dawn, however, it is revealed that the Contemelia actually instead use their Anihilaarg to create new universes as experiments.
In So Long, and Thanks for All the Smoothies, a derelict Contemelia ship which had been left automatically teleporting between universes arrived above Earth, carrying an Anihilaarg.
In A New Dawn, Maltruant, followed by Ben and Rook, traveled through the timestream to a Contemelia ship within a timeless void, where three Contemelia were about to activate the Anihilaarg in the oblivion to create the prime universe. The Contemelia explained what they were doing to Ben and Rook, and tolerated theirs and Maltruant's presence on their ship as their guests; but as observers, they observed and didn't intervene, as Maltruant attempted to derail the universe's creation and use a different, modified Anihilaarg to instead create a universe in his own image while Ben and Rook attempted to stop him. After Ben thwarted Maltruant's plans, the Contemelia resumed their plans, commenting that it would seem from what they'd just seen of Ben and Rook that their universe would be the most interesting one yet. The Contemelia released their unmodified Anihilaarg to create the universe, then released the Slimebiotes (Skurd included among them), into the newborn universe to begin gathering, carrying, mixing and depositing DNA among the universe for eons, before Professor Paradox arrived to take Ben and Rook home and to take away Maltruant's parts.
The Contemelia are intelligent and wise-seeming beings, who are known to use the Anihilaarg to create new universes as experiments. But they are also completely neutral in outside affairs as 'observers,' and will not get themselves involved in any transpiring events, even when said events threaten to disrupt their experiments and create an entirely different universe to the one they'd intended.