Qurunx

The Qurunx (pronunciation: kwʌrəŋks) were creatures of sentient energy, described by the Thirteenth Doctor as "one of the great mysteries of the universe", "rare", and "enough to power planets and civilisations". They resembled large ethereal jellyfish, with long thin tendrils glowing with blue and orange light. They utilised consciousness shields as a defence, being able to disguise themselves to the observer as something that they would instinctively want to protect, such as a child. The energy that they produced could also be channelled into an energy beam capable of disintegrating planets.

One Qurunx, being transported by a "Space Train" of the Toraji Transport Network had been taken by the CyberMasters and tethered to a metal cyber-conversion planet surrounding Earth, powering it in the process. It was hidden with a "rudimentary cloaking shield". To the Thirteenth Doctor and Yasmin Khan, it appeared as a young humanoid child, due to its consciousness shield. This was until the the Doctor disabled it, revealing the true Qurunx form. Once freed, it was ordered by the Doctor to "disintegrate everything", but before it succeeded in destroying the metal planet, it was temporarily diverted into mortally wounding her instead by the Master, leading to her regeneration. (TV: The Power of the Doctor)

Behind the scenes
It is unclear whether the "Toraji" Transport Network was intended as a reference to the similarly named Torajii Star and System (TV: 42), due to the slightly different spelling.