The celestials are a race highly advance extra-terrestrial cosmic beings who predates the dawn of the universe itself. Highly powerful, and deadly they appear in RWBY: Infinity War when Thanos had collected the Infinity Crystals and had "destroyed" the Cosmic Gods of the Universe and Reality.
According to Raven, the Time Crystal revealed to her that these Celestials are harbingers of both life, death, creation and destruction in the vastness universe, and possible in the multiverse. These beings created entire planets and galaxies in the universe, becoming the harbingers of creation. Soon afetr, they visit planet to planet in earlier stages of existence, and meddle with the DNA and genetics of lifeforms so they can evolved to become saptient and sentient with higher intellectual processing, thus harbingers of life. However, these beings also destroy planets and entire galaxies if they see the lifeforms as unworthy to continue existing, thus harbingers of death.
During Infinity War, the Celestials appear before Thanos to confront his treachery of endangering the Universe, but by the full empowered Aedra Gauntlet, Thanos effortlessly destroys the Celestial Seekers.