Rift creatures, also known as Entities, are mysterious, malevolent lifeform featured in Voltron: Legendary Defender. Initially featured in flashback episode The Legend Begins, they revealed to be overeaching antagomist of the series, having responsible for corrupting Zarkon, Honerva and became the latter's source of dark powers.
Rift creatures are formless, trans-dimensional creatures which, according to Honverva, only known lifeform that can survive passing through the Rift between worlds without use of ships made of Trans-reality Comet. One of these creatures was contained by Honerva not long after receiving signals the Altean alchemist sent into the Rift. From what reality these creatures originated from is still a mystery, but they are implied to be originated from Quintessence field as many of them surrounded the original Paladins as soon as they enter the realm due to tricked by Zarkon who desperated to save Honerva.
While weak on their own, Rift creatures pose genuine threat to any universe they come across. A single creature can summon a swarm of its brethren from the nearby opened Rift where they then combine into a giant polymorphic beast capable to fire energy blasts as well as assuming different forms to suit its attack or defense requirements. The polymorphic beast can also launch a part of itself which then became independent creature of their own.
The reason why the rift creatures referred as dark creatures are likely due to their inherently hostile nature and indication to be comprised of corrupted Quintessence; they are only know lifeforms that capable of passing through the rift between realities and Quintessence field without need of ships made of trans-reality comet, and exposure to these creatures induces physical enhancements and mental corruption similar to Quintessence itself as seen in what happened to Zarkon, Honerva, and in lesser extent, Lotor (as he was within Honerva's womb when her dying body exposed to the rift creatures). Should those who exposed to these creatures possesses the potential to magical and alchemical abilities, they can utilise rift creatures inside them as source of their powers and medium to control others, as demonstrated by Honerva aka. Haggar.
A solitary specimen was temporarily contained by Honerva, but became agitated and summoned more of its kind through the rift. They merged into one giant entity that then wrecked havok on Daibazaal, forcing Alfor, Zarkon, Gyrgan, Trigel, and Blaytz to fly out and confront it with the newly constructed Lions. It was their first battle as Paladins, and the first time Voltron was formed.