Short answers, pulled from the story.
The Tarrasque first appeared in the Monster Manual II on the 1st of January 1983. This creature was designed as the ultimate obstacle for high-level adventurers and stands 60 feet tall and 80 feet long. It is described as a force of nature that embodies wanton destruction and is immune or resistant to most offensive magic.
The Blood War is an eternal conflict between the tanar'ri demons and the baatezu devils that was introduced in 1991's Monstrous Compendium Volume Outer Planes Appendix. This war was thoroughly detailed in various books throughout the Planescape setting and updated in the 4th edition of D&D's Manual of the Planes. Different game books attribute the conflict to primordial battles between law and chaos or to Asmodeus's goal of usurping divine power.
TSR eliminated most references to demons and devils from the second edition of the game and introduced the terms baatezu, tanar'ri, yugoloth, and gehreleth to replace the original terms. Wizards of the Coast reinserted many of these references in the third edition of the game while keeping the new terms to appeal to both older and newer players. The yugoloths were called daemons in 1st edition D&D and are now listed as neutral evil fiends under their original names in fifth edition.
The kythons were created when a group of fiends called the Galchutt from Monte Cook's Chaositech and Ptolus were trapped on the Material Plane. These fiends tried creating more of their own kind through magical means, resulting in eyeless reptilian creatures with insectoid features. None of the kythons are loyal to the fiends that created them, and they are also called earth-bound demons because they originated on the Material Plane instead of the Abyss.
The current hierarchy of kythons from the weakest to the strongest is broodlings, juveniles, adults, impalers, slaymasters, and slaughterkings. As the kythons mature, they take on varied forms and eventually grow into newer and more powerful forms that closely resemble xenomorphs. These creatures are only interested in eating and breeding and have spread rapidly across the Material Plane.